I assume you had some program running that was using the speakers when it froze.
It sounds as if it freezing up
1. Overheat due to faulty / blocked fans
2. PSU voltages
3. Household power surges fridge etc switching on/off
4. faulty memory stick try memtest click here
5. virus or spyware scan for both especially if shutting down with in a few minutes of switch on or internet use
6. driver conflicts especially grahics cards get the latest drivers for your equipment