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I experimented with fitting as much audio onto a floppy last year, and got 5min 8seconds @ 7813Hz by pulling all stops (routine is just fast enough to handle ~30KHz).
Getting more than 50% compression ratio is probably not worth it on plain A500. At 20KHz you only have around 350 CPU cycles/sample, and if you're streaming from disk you also need to factor in the time to read and decode each track.
I used (slightly modified for speed) ADPCM (50%) with Huffman coding for each nibble (~92% so ~46% total), and an alternate MFM encoding scheme @ 164 tracks increasing disk capacity to 1.06MB.
Challenge is that audio data (even delta) has a lot of entropy. To really improve on the ratio you probably need to use some advanced coding scheme, but the realtime constraint really puts a limit on how much you can do, and I'm not aware of any that will work.
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