- Shortened player shoot sound from 0.5s to 0.25s sweep
- Added classic Space Invaders alien march with 4 descending bass notes
(G#1, G1, F#1, F1) matching original 1978 arcade frequencies
- Alien march tempo increases as more aliens are destroyed
- Aliens strobe brighter on each note played (0.25s brightening)
- Added explosion sound effect: white noise boom fading 0.25s
- Explosion triggered on player missile hitting alien
