16 channels for music is a lot!
I don't really know the nitty gritty of how this engine works, but could you offload music to a prerender, and then play it back on one channel as a submix? Maybe do a similar thing with the sound effects from ally javelins?
I dunno, sound in games is a foreign art to me, I come from a music production and radio production background, with a little film work, so I am sure you guys have considered all this stuff.
Still, I have spent more than a reasonable amount of time watching videos like this, https://www.youtube.com/watch?v=zF_jcrTCMsA