I've wanted to just hear the game normally during Trevor's ability since the Xbox 360 days and now I've gotten it to work while just messing about with the files one day.

This modifies Michael and Trevor's special abilities to make them a bit more powerful.

Bullet Time now reduces damage to Michael by 75 percent, letting him survive in gunfights longer and take an explosion without dying, so long as his Health and Armour is topped up.
Michael also receives a 3x damage boost to help his firearms destroy vehicles and make shorter work of his targets.

Red Mist for Trevor now works in real time, I originally wanted to give him a 25 percent speed boost, but that didn't seem to work, the changes are still on file, but the timescale doesn't seem to increase.
An unintended, but welcomed side effect to the timescale change is that now all sfx are played during Red Mist, so you'll hear Dialogue, Gunfire, Explosions and what not during the ability.

Use OpenIV to install
/OpenIV/GTAV/mods/x64a.rpf/data

This won't work with anything that edits the "playerspecialabilities.ymt" file

I haven't encountered any issues so far, entering the weapon wheel during Red Mist will cause the sfx to play and go away as if deactivating it when you close the weapon wheel, but there's no effect on gameplay.

Highly recommend using this if you're having issues with the special ability skill progress resetting
https://www.gta5-hub.com/scripts/fixed-special-abilities