RF / Scheduled playback outputs left channel only and exhibits USB audio instability after upgrade to FPP 9.3.3 / 9.4.x

[SarahDods]

Environment
Hardware
Raspberry Pi 3A+, no cape. Show master
USB Audio CODEC (stereo)
External audio processor / splitter. One to amplifier with power relay, other to FM broadcast with power relay
Two passive speakers (stereo)
GPIO relay board (used to switch amp/speaker power), driven from Pi pins directly

Software
FPP 9.3.3 initially, later upgraded to 9.4.x
Remote Falcon plugin enabled
xLights used for sequencing (not implicated)

Media
Stereo MP3 files (confirmed via ffprobe)
ALSA reports stereo output correctly

Describe the bug and Steps to reproduce:
After upgrading to FPP 9.3.3 at the start of the season (and later to 9.4.x), we encountered persistent audio issues that only occur during scheduled / show / Remote Falcon (RF) playback. Manual playback of sequences behaves correctly.

In addition, USB audio behaviour changed after the upgrade, contributing to confusing symptoms including undervoltage warnings and audio hangs at track boundaries.

These issues were not present in previous seasons with the same hardware and persist across upgrades.

Issues Observed (Chronological)

1. After upgrading to FPP 9.3.3, behaviour differed from previous seasons (including needing adjustments to some GPIO/relay control scheduler commands and scripts; not the focus here).

2. After upgrading to FPP 9.3.3, USB audio behaviour appeared more power- and timing-sensitive. During audio playback, Raspberry Pi undervoltage warnings were observed and audio would sometimes hang at track boundaries. Replacing Pi power supply with higher current eliminated undervoltage warnings but not crashing. Lowering digital audio output from 90 to 70 resolved the hanging behaviour, suggesting a relationship between USB audio handling, power draw, and playback stability.

3. Manual playback of sequences via the FPP UI outputs stereo correctly on both speakers (which makes debugging a bitch).

4. Scheduled / show / RF playback outputs audio on the left channel only, with the right channel silent.

5. The above symptoms do not occur when manually playing a sequence, only during RF/show playback. That plays stereo.

6. Media files are confirmed stereo and play correctly outside RF/show mode.

7. Re-exporting sequences and verifying xLights configuration had no effect.

8. Upgrading from FPP 9.3.3 to 9.4.x did not change the behaviour.

9. Attempts to correct RF audio in software (VLC options, ALSA upmixing) either:

• had no effect on channel output, or
• caused fppd startup instability or very slow UI, requiring a full power-down to recover.

10. Manual/test playback and RF/show playback clearly use different audio paths; no supported FPP UI or RF plugin setting exists to force mono-to-stereo duplication in RF/show mode. The only reliable workaround found was hardware mono duplication (wiring both speakers to L amp channels).

Expected behavior:
RF/show playback should:

• preserve stereo output when the source media is stereo, or
• provide a documented and supported way to duplicate mono program audio to L/R.

USB audio handling should not introduce undervoltage conditions or cause audio to hang at track boundaries.

Audio initialisation failures should not block fppd startup or UI responsiveness.

Additional context:
This issue is not related to xLights or sequence configuration.
ALSA and hardware output are confirmed stereo.
Manual sequence playback behaves correctly.
Issues appear isolated to the RF/show audio pipeline combined with changed USB audio behaviour.
Thank you ChatGPT for supporting ~20 hours of debugging to get this far.

Additional Attachments
FPP_Logs_20251215.1209.zip