The st had midi built in so you could connect it to pro music equipment out of the box. Thus the preception of better audio.
Just like the Mac had the ability to integrate with Adobe postscipt and laser printers, hence the poplularity of the Mac as a DTP solution...
The Amiga had it's video abilities so it became the DSKTOP video leader for a time.
The ST Falcon actually has a sound DSP at 16 bits so TECHNICALLY yes some of the ST's have better sound compared to the amiga as the sound hardware on the amiga never got the dsp chip. The aA3000+ was slated to have the chip included but that never happpened.
BUT the vast majority of games use the ST's builtin sound chip (not MIDI, not 16 bit dsp) as the Flacon only saw limited release compared to say the A1200/A4000.
