Will channel:setVolumeMod() with sound.envelope work? I've been panning sampleplayers with channel:setPanMod() to good effect. The sound is added to a channel with addSource and then the channel can be controlled all kinds of ways beyond what the sample alone can do.
(I found some bugs that are being fixed—but they may apply only to synths? Anyway: find workarounds here in the meantime if needed.)