I was wondering if there was something that I could use in order to play my favorite music out of my amp while playing to it. I noticed most Marshalls have that built in, but my current amp doesn't... unless I'm missing something.

Anyway, is there such a tool in this world?
You may be abe to plug straight into the power amp if your amp has an effects loop, use splitters into the input etc. Should be fine if your amps solid state, i do it in school constantly... Otherwise I wouldnt reccomend it.
if your amp has 2 inputs get a headphone cable to guitar cable converter and a RCA to headphone converter so u can plug in ur portable CD/MP3 player...its gonna sound like **** though
Only good on a solid state? So on a tube amp it would sound ****ty?

I have the 1/8 to 1/4 converter.

Bair, waht would you need the RCA converter for?

My friend has a MArshall solid state practice amp with a cd in and it sounds perfectly fine, i.e. distortion doesn't affect it.
What I'm getting so far is there's no switch or tool in existence, huh.

What kind of connectors are ofund on the back of cabinets? More than 1 input?