Convert M4A to OGG
Drop an .m4a file and download an .ogg file carrying Opus audio at 128 kbps. Sample rate and channel count pass through unchanged.
The reason to do this is a destination that names the format: a game engine’s audio importer, a Linux packaging requirement, or a web project standardizing on Opus.
Two lossy codecs
AAC decodes to PCM, then Opus encodes it again. Opus is efficient enough that 128 kbps generally holds the AAC source without audible further damage, but the theoretical ceiling is still the M4A, not the original recording.
Encode from a WAV or FLAC master when one exists.
Speech content
Opus handles voice better than either MP3 or AAC at low bitrates. For an interview or narration track pulled out of an .m4a, dropping to 64 kbps in the full Audio Converter often produces a much smaller file with no practical loss of intelligibility.