iOS 16: fix playback for podcasts with a single channel + Spatial Audio enabled

[leandroalonso]

Fixes #62

iOS 16 seems not to like the combination of mono audio (single channel) and Spatial Audio enabled. When those two conditions are met and EffectsPlayer is used the audio just doesn't work.

This seems to be an issue on Apple's side, but given they might not fix it by the time iOS 16 is released, it is good to handle it on our app.

The change works in this way: if the user is on iOS 16 and the user is playing a file with one single channel, we convert the buffer to 2 channels to avoid this issue.

To test

There are a few ways to test this PR, by manually adding your own files and by testing with podcasts. But before testing, make sure that:

1. You're using a real device with iOS 16
2. You're using an AirPods Pro/Max or any other device with Spatial Audio capabilities
3. Enable Spatial Audio: when playing audio on Pocket Casts, swipe down to access the control center, tap the audio bar and hold, tap "Spatialize Stereo", choose "Fixed" or "Head Tracked"

All done? Let's go.

Testing with a podcast

1. Search for the podcast "The Rizzuto Show"
2. Download any episode
3. Wait for the download to finish
4. Play it
5. ✅ Make sure the podcast play correctly

Testing with audio files

You can use your own audio files with one or two audio channels, or you can download some here: https://espressif-docs.readthedocs-hosted.com/projects/esp-adf/en/latest/design-guide/audio-samples.html

1. Add a file that has two-channel audio
2. Play it
3. ✅ The audio file should play correctly
4. Add a file with a single-channel audio
5. Play it
6. ✅ The audio file should play correctly

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[emilylaguna]

Wow, 👏 👏 bravo, very nice fix. I tested using my own mono audio files and the test podcast and reproduced the issue then switched to the branch and all is working now.

I left a comment about using some force unwrapping and whether we need to do it.

[emilylaguna]

audioPCMBuffer!.audioBufferList.pointee.mNumberBuffers

Do we need to force unwrap here?

[leandroalonso]

Ops, we don't. Removed in 1a5cb6b

[emilylaguna]

Nice work!