Hello specvmama and welcome to the Nissanhelp.com forums!!!
Nissan has lots of problem with audio system. I check the wire diagram for your Sentra, the signal to the speaker comes from the radio trough the speaker amplifier. All the signal are individual and the sub-woofer signal is taken from the rear speakers. You can test for signal from the radio to amp, but most likely the amp is bad. Unfortunately the amp is too expensive and it may not be worth replaced. There is a company that can repair the amp for your, the web site is:
http://carstereohelp.com