@zing many thanks for such a detailed response. For now I have put the IPAC in to dual controller mode and everything seems to be working fine. I will work my way through those links to find a more permanent solution.
@gkralicek2 To my understanding, it's a "different story". The dreamcast was outputting sound at ... i don't know ... say 48kHz. If your hardware is only capable of 44.1kHz, then comes down sampling. Alsa is supposed to handle that. And it's supposed to handle any sound distortion (otherwise the downsampling algorithm is a failure ...). Still, I'd be more curious to see if the same 20% retard exists on PC too with reicast, which would helpfondong if it's a hardware problem or an emulation problem.