MP3 / Opus playback still broken with system ffmpeg

Description:

Despite various fixes in Qt 6.8.1 for QTBUG-130273 I still see (at least) two issues with codec handling:

MP3: While direct file playback works, playback via WebAudio fails with "Failed to execute 'decodeAudioData' on 'BaseAudioContext': Unable to decode audio data". This can be reproduced on https://mdn.github.io/webaudio-examples/decode-audio-data/promise/ where clicking "Play" logs that error and doesn't actually play anything. The same also happens when notification sounds (are supposed to) play on lichess.org.

Opus: Playback still seems to be broken despite the fix: https://tools.woolyss.com/html5-audio-video-tester/?u=woolyss.com/f/audio-sample.opus has the play button greyed out. The same also happens when trying to play WhatsApp Web voice messages (clicking the play button does nothing).

For Opus, chrome://media-internals shows:

"Warning, FFmpegDemuxer failed to create a valid/supported audio decoder configuration from muxed stream, config:codec: opus, profile: unknown, bytes_per_channel: 0, channel_layout: MONO, channels: 1, samples_per_second: 48000, sample_format: Unknown sample format, bytes_per_frame: 0, seek_preroll: 80000us, codec_delay: 104, has extra data: true, encryption scheme: Unencrypted, discard decoder delay: true, target_output_channel_layout: NONE, target_output_sample_format: Unknown sample format, has aac extra data: false" "FFmpegDemuxer: skipping invalid or unsupported audio track" "FFmpegDemuxer: no supported streams"

Passing --disable-features=FFmpegAllowLists fixes both of those issues, but the allowlist seems to be there for security purposes.

Perhaps Archlinux should just go back to using the bundled ffmpeg, like Alpine, Debian, Gentoo, ...?

Additional info: