General Increase:
* Increase
* Rise
* Growth
* Escalation
* Build-up
* Amplification
* Intensification
Musical Context:
* Forzando (a sudden increase in volume)
* Swell
* Climax (the highest point of intensity)
Figurative Language:
* Surge
* Wave
* Whirlwind
The best synonym depends on the specific meaning you want to convey.