I actually reported it on Trello myself some time ago. Fun fact: On UGMP the custom radio stations are actually quieter than the normal SA stations.
Joined on Feb 12 2021
Posted a quarter of a year ago
That is quite odd. It's possible that the game's volume controls are not working properly. However on older versions when the radio stations were mp3/wav format, this was still a problem.
Also, any possible chance that the radio zones could be parameterized? I absolutely hate how the stations only play when your right next to the city, and the San Andreas ones play everywhere else, except the added cities.