When it comes to the in-game settings, there isn't much to do. The Audio settings don't have anything relevant; just make sure your PC is using the SteelSeries Sonar - Gaming as the audio device, and in Sonar, select the device that the channel is using (as in your headset or speakers). This ensures that the Preset you selected for the Game channel is being used.
Under Voice Chat, you can select the appropriate devices managed by Sonar: SteelSeries Sonar - Chat for incoming audio chat, and SteelSeries Sonar - Mic for your voice.
This way, you can manage the volume of voice chat separately from the game audio by moving the slider in Sonar. For your Mic, there are also presets you can apply to your voice if you want — Walkie Talkie is a fun one.
How to get the best GTA V audio on console