So it turns out that, even though the ulaw encoding / decoding is working fine, it is not great for the drums - the encoding causes the noise floor to raise enough that you hear a 'hiss' or static-like sound when a sample is playing at very soft levels. Unfortunately, this happens a lot, especially for longer cymbal sounds like the ride cymbals.

At this point I have removed all the ulaw encoded sounds, in favour of raw 16 bit PCM. With the new 128MB flash chip I have enough room for all the samples that I need (plus a bit of headroom), so it's working just fine.