[postprocessor/ffmpeg] Allow options for mp3

[a-rich]

Why?
Options like preferredquality are ignored when postprocessing mp3s.

What?
Remove mp3 from the list of filecodec values to check for in the "lossless" condition.

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Description of your pull request and other information

Why?
Options like preferredquality are ignored when postprocessing mp3s.

What?
Remove mp3 from the list of filecodec values to check for in the "lossless" condition.

[dirkf]

I'm not convinced about this.

The existing logic tries to avoid re-encoding the audio unless the target container requires it. As I see it, this change on its own will generally result in worse audio quality than what the site offered: if you ask for lower quality you'll get it and if you ask for higher quality you'll get a degraded version of the original with a higher nominal bitrate. Unless you always want to dumb down the MP3 quality to reduce filesize, that doesn't sound good.

You could try this

-            elif filecodec in ['aac', 'flac', 'mp3', 'vorbis', 'opus']:
+            elif filecodec in ['aac', 'flac', 'vorbis', 'opus'] or (filecodec == 'mp3' and self._preferredquality is None):

If you specify --audio-quality ..., that should be passed into self._preferredquality and cause the re-encoding that you want to occur. A more complex change could extend the same processing to the other audio formats.

The FFMPEG audio option processing is definitely a target for updates. yt-dlp has some more sophisticated processing but I haven't studied it fully.

[a-rich]

I see. I've amended the commit to incorporate your suggestion which I feel achieves both aims...

if you ask for lower quality you'll get it
which satisfies my need to ensure a standardized bit rate

and [preventing asking] for higher quality [and getting] a degraded version of the original with a higher nominal bitrate
which avoids the needless and unexpected audio degradation