Audio Volume Balancing and Mixing

## Description
Sound effects have inconsistent volume levels. Some sounds are extremely loud and uncomfortable, while others are barely audible, even when they should be noticeable.

## Expected Improvement
- Normalize sound effect volumes
- Establish consistent loudness ranges for:
- Weapons
- Environmental sounds
- Enemy sounds
- UI feedback
- Test audio using both headphones and speakers

## Notes
- Consider using Unity’s Audio Mixer for better control
- Add simple volume sliders for future polish

Look for Unity Audio Mixer assets and sound effect files in the project. Start by opening the Unity project, finding the Audio Mixer, and examining existing sound effect volume levels. Test with headphones and speakers to identify inconsistencies. 'Done' means normalized volumes across categories (weapons, environment, enemies, UI) and possibly a basic volume slider setup.

Written by the indexing model from the issue text.