How do I control the volume on my audio/video sys for my van. It has an FM modulator and no manual to help?

Its a CM-701DR Monitor with a remote that doesn't control volume. It has built in speakers but without a manual I can't figure it out. It did come with cordless headsets and one of them does work to hear the sound,but thats it. The movie plays on the screen but no sound. Does the fm modulator have something to do with tuning a certain channel on the radio to hear? I did go through just about every frequency and that didn't work either. Well thanks for any advice.How do I control the volume on my audio/video sys for my van. It has an FM modulator and no manual to help?
Try pushing the shag carpeting aside and turnig the volume knob.... that always worked for me in my van back in the day.How do I control the volume on my audio/video sys for my van. It has an FM modulator and no manual to help?
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If it has an fm modulator, try this. Turn on the set and play a movie. then hit the scan/seek selector on your radio. It will probably stop somehwere around 88.1 and your sound will play over the vehicle stereo system......... Note: Frequencey may be anywhere between the 80's and 90's . Oh and you will probably have to plug it in to your radio antenna, some are required to do that. Some transmitt a wireless signal. Look at the modulator if it must be plug to the antenna there will be two wires, one male one female. the male side would plug to the rear of the vehicle stereo and the female side to the antenna coax, which you would disconnect from the radio.....