I think that won't work because the beep sound not coming from the radio.I don't have blind spot alert on my 2015 GLA, but wonder if what I did to resolve a problem with Becker Map Pilot might help.
I noticed that the Becker audio directions were too quiet to hear over music, but couldn't find anything in the manual that addressed volume change. By trial and error I finally figured out that you could change the Map Pilot audio volume ONLY when it was making an announcement. Once the volume adjust was made it would be at that volume from then on until you changed the volume during a future announcement.
1. Turn your music volume to a relatively low setting
2. Drive into a position where the blind spot warning will activate
3. While blind spot warning is on, use the steering wheel volume button to increase sound level