External memory, make samples a big as you want.
2 seconds of drum sound forced to decay off to fit in memory, sampled at 32K to achieve 16KHz bandwithd, is 64K of memory. Adds up quick! And not realistic sounding either.
Go with external memory, read back the samples and feed into separate D/A converters and mix together in analo world.
I have a design I did, because I wanted to go electronic and stop lugging my set all over, where I used a PC to load up sounds into shared memory, then had my playback circuit read it back from memory thru D/As. Trigger was a microphone inside a practice pad, hardness of the strilke was captured used to control volume of playback by having the D/A output go thru a voltage controlled ampifier.
Had all the pieces prototyped, tested with components that gave good results, had a method to load up banks of memory using a PC Junior (this was like 1987).
Then we packed up to buy a house, things got delayed a few months, we finally moved, the Roland type digital drum sets came out, and Ijust never got back to doing anything with it.
Now, everything less expensive, especially memory (4 gig USB stick, $8!, surprised they are not free in cereal boxes), speeds are faster. I could see buildiing a ATMega per sound channel with meg of memory or something, build up 16 channels or something, master controller to have pc connected to each download to each one to load up memory for playback by the atmega with a strike detection circuit per card or something.
I'm gonna have to dig up my old design, update it for these new times!