Note that "It is really simple" really only applies to the STE. For STF class machines a lot more work is necessary.
I have ready made code on the Paradize homepage to use the STE DMA sound feature.
It plugs into GFABASIC (and i have a version for AHCC too) but the main work is done in M68K.
Check the GFABASIC section at http://paradize.atari.org/