Flipper/Wave_Player

Needs the WAV Player application compiled into firmware.

Original code from Zlo and most files provided by RogueMaster.

Create a wav_player folder in the root of your SD card. You'll need 8-bit unsigned WAV files (try Audacity for conversion.)

WAV Player is apparently very particular about the format. WavPlayer wants BitExact, NO headers, 48k PCM unsigned 8-bit stereo. Example:

Input #0, wav, from '.\canyon.wav':
Duration: 00:02:03.38, bitrate: 768 kb/s
Stream #0:0: Audio: pcm_u8 ([1][0][0][0] / 0x0001), 48000 Hz, stereo, u8, 768 kb/s

To produce a compatible file, you can also use FFmpeg:
ffmpeg -i .\input.mp3 -c:a pcm_u8 -fflags +bitexact -flags:a +bitexact -ac 2 -ar 48k output.wav