Is there a good compressed audio-format (like MP3) that i can use on my 386?
Intel 80386DX-20 @ 40Mhz
coproc.
It has 20megs of ram.
it runs DOS 6.22 / Windows 3.11 and Windows NT 3.51.

You dont have enough processing power.

If you still want to experiment with such a machine I suggest you try to encode with the DOS program called AMPEG.
This program encodes to Layer I and II audio (.mpg,.mp2).

Maybe if you encode a 8 kHz mono .wav file to Layer I at high bitrate (less compression) you will succeed to play it in realtime.
Quality will be very very poor.
If you are able to get higher quality please post it here! ;-)

I suggest you upgrade your processor to at least a 486 DX 66.

If you can find a hardware MPEG1 video card you will be able to play the encoded Layer I and II files in 44 Khz stereo with that.

If you can find a hardware MP3 card that would be even better.

Of course Audio CD's, uncompressed files (like .wav), MIDI and maybe even Amiga MOD files can be played with your system...

You might want to try a DOS command line based MP3 player. What da heck! Download DOSAMP and DOSAMPEX

http://members.cox.net/dos/sound01.htm

Even with the DOS players (MPX Play performs well if no graphics are used) the processing power is not sufficient....
WinPlay3 and Audio Active perform very well too on my 3.1x systems.

Do you want a audio format to save sounds
in your disk?
Try IMA ADPCM, it is a compressed wav format
and can be uncompressed/played in realtime.
(of course the compress rate is not as good
as MP3)

You can look up my page under Imaging/multimedia to find several older sound player for a 386, including Dosamp. But my guess is the best you can hope to write and read with any efficiency is the MP2 format, forget MP3!