I've been having a play would some sound effects just now, and for basic TED sounds.... its not bad really. I can only use one of the TED channels since I need to change the volume to create an ADSR envelope. Its a pitty since 2 channels would me a big difference to the overlapping nature of effects. Still, I've added in apriority system so firing sounds are drowned out by explosions, which in turn are drowmed out by pickups and so on. It means that at least theres some basic sound effects as well as music - normally you only get one or the other.
I've had to create a 64 byte volume table to allow the sound effect volume to be turned up/down. Since I use the volume to create the ADSR, I need a redirection table depending on what the volume actually is. For example, if we're at volume 4 then ADSR value of 4 becomes 2 (instead of staying 4 when SFX volume is 7). This means I can turn down effects smoothly just like the music.