Sub-woofer operation

Question

I have an old set of GREAT Yamaha computer speakers including sub-wooferYST-M15. The sub-woofer doesn't stay on if not in use for a while. However, when I play a .wav , etc. file it USUALLY turns on. Is it triggered by the sound or do I have a short in the on-off button. I've never had a sub-woofer like this. It doesn't turn on for very short .wav files. Please e-mail me [e-mail] and thanks for your help!