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