I want to listen to music through my guitar amp, but it always cuts out the right channel completely I don't want to have to move the music file into an audio editing program to make it mono every time I want to listen to music through the amp (can't afford a new stereo). Are there any programs like Windows Media Player or Itunes that have an option for mono playback? Thanks!
What??? It does that?

I just plug anything that plays music e.g Laptop, Mp3 player etc from the 3.5mm jack to a 6.3mm (or whatever that standard amp input is cbb looking it up (with a coverter plug obvs)) into the "CD IN & Emulated line out" socket.. Or just the normal input, but it sounds better in the former. Sounds normal, a little bassy' but eh.

Edit: My amp is a stack so... Maybe I do have stereo? Idk this sh!t.. sorry. haha
I don't know about most devices but since you didn't mention what you're trying to listen to I'll try to help you out with what I know. If you have an iPod touch or iPhone go into settings > general > accessibility and you should be able to turn on mono audio.
I'm using Windows Media Player, and I don't have an Ipod. I've looked into the settings in WMP, but I can't seem to find anything that would change it into a mono output. My amp unfortunately doesn't have RCA inputs I will look into finding an adapter for stereo to mono that might help me out, thanks!
Not sure if this would work but:
iPod/computer/whatever --> Male 3.5mm to female RCA --> RCA to RCA --> Female RCA to male 1/4"-->amp.
With this method you'd be taking the audio and splitting it into stereo in the separate RCA cables, then it would merge into mono, theoretically.