Almost 1 year later, and it seems this is still an issue.
I've bought a Packtalk Edge last week, so here's my experience so far.
When I'm on my bike, I'm running the Flitsmeister & a separate navigation app.
The first time I've tried my Packtalk, I was using Tomtom Ride, and Spotify, which seemed to work fine.
Last weekend, I started with just Flitsmeister, and MRA Next. Volume was really low, just heard some very muted whispering from the nav. app. Flitsmeister was quite loud.
On a given moment, I've got a FM notification, simultaneously with a MRA notification, and then the volume was suddenly OK. Yet, only this moment, and not afterwards.
So I've did some searching, and found this thread. However, I can't find the setting as in the screenshot above?
Anyway... The next day, tried to run JUST MRA Next, hoping it would sound OK, without any other apps interfering. But no cigar... So I had a F* it moment, and turned on Spotify to just play some music, and guess what... Volume was OK.
Nice to have it partially working, but still looking to have some decent volume without music.