The only way I found for my custom ringtone to work while USB is connected (and to prevent losing my setup if a call or an alarm happens) was a tip from here (vote him up please).
Basically (with some extra info):
1) you need a rooted device (for m-mp5303g the only method that worked was by using cydia impactor
# drop SuperSU su to /system/xbin/su and do not upgrade SuperSu after that, and on linux this was also required
sudo adb kill-server;sudo adb start-server)
2) I used use X-Plore with root write access option enabled to copy the ringtone
3) find the alphabetically first ringtone, rename it to a
.old file, rename your custom ringtone to that ringtone filename. This will help in case some ringtone configuration is lost, that file will be used as default (in m-mp5303g at least). Obs.: make it sure you do not care for that first ringtone music.
Obs.: this seems to work with alarms and notifications too.
PS.: just adding files there did not work here, they all were ignored, even after reboot; only replacing worked.