As you have found, driving a phonograph input with anything other than a magnetic or
ceramic cartridge doesn't work. A magnetic pickup wants something in the 47,000
ohm range and a
ceramic would prefer 500,000 ohms or more. The headphone output
jack is typically 32 ohms. The massive mismatch is part of the problem. Another issue is that phonograph inputs have RIAA equalization, the input will boost the low frequencies and cut the high frequencies. This will make the audio from any other source than a vinyl record sound odd if not just plain unlistenable. You will still have an
impedance mismatch with a regular AUX input but its much smaller and you can usually get away with it.
Your best bet is to find a
RCA switch that will let you use another input with 2 or more devices. They start at $10 on Amazon. They will also
switch video but you don't need to
plug anything into the video jacks, you can just use the audio switching. And don't forget to run the volume up on the
MP3 player. I tend to turn it up all the way then back it down a notch or two as less expensive players will overdrive the output at full volume which causes
distortion and is hard on things.
Michael