diff --git a/lib/media/media_source_engine.js b/lib/media/media_source_engine.js
index b638b964e3..f2c8a84aa3 100644
--- a/lib/media/media_source_engine.js
+++ b/lib/media/media_source_engine.js
@@ -280,7 +280,7 @@ shaka.media.MediaSourceEngine = class {
         if (contentType == ContentType.VIDEO) {
           codecs = StreamUtils.getCorrectVideoCodecs(codecs);
         } else if (contentType == ContentType.AUDIO) {
-          codecs = StreamUtils.getCorrectAudioCodecs(codecs);
+          codecs = StreamUtils.getCorrectAudioCodecs(codecs, mimeType);
         }
         const extendedMimeType = MimeUtils.getExtendedType(
             stream, mimeType, codecs);
diff --git a/lib/transmuxer/ts_transmuxer.js b/lib/transmuxer/ts_transmuxer.js
index 1fb418c8d0..c8a65c4b9e 100644
--- a/lib/transmuxer/ts_transmuxer.js
+++ b/lib/transmuxer/ts_transmuxer.js
@@ -144,13 +144,17 @@ shaka.transmuxer.TsTransmuxer = class {
   convertCodecs(contentType, mimeType) {
     if (this.isTsContainer_(mimeType)) {
       const ContentType = shaka.util.ManifestParserUtils.ContentType;
+      const StreamUtils = shaka.util.StreamUtils;
       // The replace it's necessary because Firefox(the only browser that
       // supports MP3 in MP4) only support the MP3 codec with the mp3 string.
       // MediaSource.isTypeSupported('audio/mp4; codecs="mp4a.40.34"') -> false
       // MediaSource.isTypeSupported('audio/mp4; codecs="mp3"') -> true
       const codecs = shaka.util.MimeUtils.getCodecs(mimeType)
           .replace('mp4a.40.34', 'mp3').split(',')
-          .map(shaka.util.StreamUtils.getCorrectVideoCodecs).join(',');
+          .map((codecs) => {
+            return StreamUtils.getCorrectAudioCodecs(codecs, 'audio/mp4');
+          })
+          .map(StreamUtils.getCorrectVideoCodecs).join(',');
       if (contentType == ContentType.AUDIO) {
         return `audio/mp4; codecs="${codecs}"`;
       }
diff --git a/lib/util/stream_utils.js b/lib/util/stream_utils.js
index 5e48afd5bd..98eaf7845d 100644
--- a/lib/util/stream_utils.js
+++ b/lib/util/stream_utils.js
@@ -439,7 +439,8 @@ shaka.util.StreamUtils = class {
         videoCodecs = StreamUtils.getCorrectVideoCodecs(videoCodecs);
         let audioCodecs = ManifestParserUtils.guessCodecs(
             ContentType.AUDIO, allCodecs);
-        audioCodecs = StreamUtils.getCorrectAudioCodecs(audioCodecs);
+        audioCodecs = StreamUtils.getCorrectAudioCodecs(
+            audioCodecs, video.mimeType);
 
         const audioFullType = MimeUtils.getFullOrConvertedType(
             video.mimeType, audioCodecs, ContentType.AUDIO);
@@ -475,7 +476,8 @@ shaka.util.StreamUtils = class {
     }
 
     if (audio) {
-      const codecs = StreamUtils.getCorrectAudioCodecs(audio.codecs);
+      const codecs = StreamUtils.getCorrectAudioCodecs(
+          audio.codecs, audio.mimeType);
       const fullType = MimeUtils.getFullOrConvertedType(
           audio.mimeType, codecs, ContentType.AUDIO);
 
@@ -764,17 +766,18 @@ shaka.util.StreamUtils = class {
         // We ignore the multiplexed audio when there is normal audio also.
         if (videoCodecs.includes(',') && !audio) {
           const allCodecs = videoCodecs.split(',');
+          const baseMimeType = MimeUtils.getBasicType(fullMimeType);
 
           videoCodecs = ManifestParserUtils.guessCodecs(
               ContentType.VIDEO, allCodecs);
 
           let audioCodecs = ManifestParserUtils.guessCodecs(
               ContentType.AUDIO, allCodecs);
-          audioCodecs = StreamUtils.getCorrectAudioCodecs(audioCodecs);
+          audioCodecs = StreamUtils.getCorrectAudioCodecs(
+              audioCodecs, baseMimeType);
 
           const audioFullType = MimeUtils.getFullOrConvertedType(
-              MimeUtils.getBasicType(fullMimeType), audioCodecs,
-              ContentType.AUDIO);
+              baseMimeType, audioCodecs, ContentType.AUDIO);
 
           audioConfigs.push({
             contentType: audioFullType,
@@ -825,10 +828,11 @@ shaka.util.StreamUtils = class {
     }
     if (audio) {
       for (const fullMimeType of audio.fullMimeTypes) {
-        const codecs = StreamUtils.getCorrectAudioCodecs(MimeUtils.getCodecs(
-            fullMimeType));
+        const baseMimeType = MimeUtils.getBasicType(fullMimeType);
+        const codecs = StreamUtils.getCorrectAudioCodecs(
+            MimeUtils.getCodecs(fullMimeType), baseMimeType);
         const fullType = MimeUtils.getFullOrConvertedType(
-            MimeUtils.getBasicType(fullMimeType), codecs, ContentType.AUDIO);
+            baseMimeType, codecs, ContentType.AUDIO);
 
         // AudioConfiguration
         audioConfigs.push({
@@ -991,9 +995,10 @@ shaka.util.StreamUtils = class {
    * Generates the correct audio codec for MediaDecodingConfiguration and
    * for MediaSource.isTypeSupported.
    * @param {string} codecs
+   * @param {string} mimeType
    * @return {string}
    */
-  static getCorrectAudioCodecs(codecs) {
+  static getCorrectAudioCodecs(codecs, mimeType) {
     // According to RFC 6381 section 3.3, 'fLaC' is actually the correct
     // codec string. We still need to map it to 'flac', as some browsers
     // currently don't support 'fLaC', while 'flac' is supported by most
@@ -1008,8 +1013,16 @@ shaka.util.StreamUtils = class {
     }
 
     // The same is true for 'Opus'.
-    if (codecs === 'Opus') {
-      return 'opus';
+    if (codecs.toLowerCase() === 'opus') {
+      if (!shaka.util.Platform.isSafari()) {
+        return 'opus';
+      } else {
+        if (shaka.util.MimeUtils.getContainerType(mimeType) == 'mp4') {
+          return 'Opus';
+        } else {
+          return 'opus';
+        }
+      }
     }
 
     return codecs;
diff --git a/test/test/util/util.js b/test/test/util/util.js
index 87927d3c04..35bfe30ca8 100644
--- a/test/test/util/util.js
+++ b/test/test/util/util.js
@@ -319,6 +319,7 @@ shaka.test.Util = class {
    */
   static async isTypeSupported(mimetype, width, height) {
     const MimeUtils = shaka.util.MimeUtils;
+    const ContentType = shaka.util.ManifestParserUtils.ContentType;
     const StreamUtils = shaka.util.StreamUtils;
 
     /** @type {!MediaDecodingConfiguration} */
@@ -326,8 +327,9 @@ shaka.test.Util = class {
       type: 'media-source',
     };
     if (mimetype.startsWith('audio')) {
+      const baseMimeType = MimeUtils.getBasicType(mimetype);
       const codecs = StreamUtils.getCorrectAudioCodecs(
-          MimeUtils.getCodecs(mimetype));
+          MimeUtils.getCodecs(mimetype), baseMimeType);
       if (codecs == 'ac-3' && shaka.util.Platform.isTizen()) {
         // AC3 is flaky in some Tizen devices, so we need omit it for now.
         return false;
@@ -341,10 +343,10 @@ shaka.test.Util = class {
         // https://bugs.chromium.org/p/chromium/issues/detail?id=1450313
         return false;
       }
-      const baseMimeType = MimeUtils.getBasicType(mimetype);
       // AudioConfiguration
       mediaDecodingConfig.audio = {
-        contentType: MimeUtils.getFullType(baseMimeType, codecs),
+        contentType: MimeUtils.getFullOrConvertedType(
+            baseMimeType, codecs, ContentType.AUDIO),
       };
     } else {
       const codecs = StreamUtils.getCorrectVideoCodecs(
@@ -352,7 +354,8 @@ shaka.test.Util = class {
       const baseMimeType = MimeUtils.getBasicType(mimetype);
       // VideoConfiguration
       mediaDecodingConfig.video = {
-        contentType: MimeUtils.getFullType(baseMimeType, codecs),
+        contentType: MimeUtils.getFullOrConvertedType(
+            baseMimeType, codecs, ContentType.VIDEO),
 
         // NOTE: Some decoders strictly check the width and height fields and
         // won't decode smaller than 64x64.  So if we don't have this info (as