One of our sites just moved from Engage Mobile to the Vina app. They claim the alert tone for user to user messages was a lot louder in Engage Mobile. It would be nice if there were a setting, like alert tones in send rules, where we could chose a louder notification tone for user to user messages.
Some Vina tones are naturally louder than others and we do offer several variations of each ringtone with varying levels of audio chimes to aid in this problem. The ability to modify volume independently for direct messages (outside of device volume) is not on the roadmap for Vina but is something we are exploring as an overall enhanced user experience in future apps.