@powtrap First up its nice to hear that it works for you, but if you are using other scripts/mods which are solely based on SHVDN, where you will need the said dll, till then enjoy the game as it is. Though it's strange that it fails for you due to SHVDN, since, it never failed for me.
@Frazzlee IDK, I find it strange that SHVDN doesn't work, since, AFAIK, its just a .net version of the orignal SHV, could be wrong though.
There's a simple answer to the problem. The game radio audio is processed to sound punchy broadcast-like, where your own-sourced mp3's (or whatever) aren't.
If you look up the natives, in a very simple scripthook mod that's easily built in C++ or ScripthookDotNet mod in C#, that enables a game-builtin that raised external audio file playback level - it simply rescales it in a DRC like fashion. I believe that same native is actually used in game by the game engine to synthetically raise levels for the in-game radio and some other background music audio instances.
Alternatively, and dead easy if you don't want to build anything, make a copy of your MP3's or M4A's (the two commonly used formats supported) and use something AACGain or MP3Gain to create non-destructively rescaled/embedded ReplayGain normalised versions to use with Self Radio.
I've used both methods, but i don't use randomly assembled sets of music - for self radio I build proper broadcast ready audio segments so that all it needs is the native engaged to raise the level to bring it into balance in game (as the PPL and RMS is slightly hotter than the game radio files).
I'll leave you to a google-fest session to go find the tools and techniques, or maybe you might even find my write-up elsewhere on another forum.