Convert FLAC to M4A
Drop a FLAC and download an .m4a file carrying AAC audio at 192 kbps. Sample rate and channel count come across unchanged.
Because FLAC decodes to the original PCM, the AAC encoder sees a full-quality signal. This is one lossy generation, which is the best result any lossy format can give.
Lossless library, lossy copies
The usual pattern is a FLAC archive at home and AAC copies on the phone. Keeping the FLAC means every future device or format can be served from an unspoiled source, while the M4A handles the storage-constrained end.
AAC at 192 kbps
At this bitrate AAC is transparent enough for phone playback and car audio over Bluetooth, which itself re-encodes the stream. Spending 320 kbps on audio that a Bluetooth codec will compress again buys very little.