The Savvy API allows for customizing the audio theme, such as the default Ringtone and SMS notification sounds.
To alter these, simply copy the desired files into
src/system/custom/etc/dconf_source/db/custom.d/custom.key and look
for the following:
[com/ubuntu/touch/sound] incoming-call-sound='/custom/usr/share/sounds/ubuntu/stereo/ring.mp3' incoming-message-sound='/custom/usr/share/sounds/ubuntu/stereo/sms.mp3'
Change the paths to match the files that were just copied over.