no sound on my tv

I have an old NES system and I have no sound on the tv. I plugged in the RF switch adaptor but no sound. I tried the rca plug-ins also and there is no sound. I have another NES system and there is sound thru the RF switch adaptor so it's not the tv or adaptor. How do I fix this? What can I do to get sound? I am not sure what is exactly wrong or what to exactly look for.