"MoreAmp is an audio player, transcoder and cd-ripper for Mac OS X,
Windows, Unix, and Linux. Plays and creates OGG, FLAC, MP3, AAC, M4A,
MP4, WAV, and AIF, and plays WMA.
31-band equalizer, repeat loop, variable pitch/tempo, RAM or RAM-disk
preload, more."

#1 Download: The latest OS 9 compatible version available
(2009/08/19).
It's the only (known) FLAC player for classic Mac OS (with the
exception of [Amadeus II][1] which is more of an audio editor than a
"friendly" player). It successfully played a libFLAC 1.3.1 file on Mac
OS 9.
Currently incompatible with M4A + AAC or MP4.

MoreAmp development ceased in 2019 but "Anyone interested is welcome
to take it over.".

Compatibility
Architecture: PPC

Mac OS 9. Compatibly with 8.6 and Classic unknown.

  [1]: http://macintoshgarden.org/apps/amadeus-ii-387