Getting rid of echo effect

[Sethomas]

So, to avoid waking up people who share my room (ahem) I realized that my moniter has audio input and output jacks, such as it could be used for television and such. (Yeah, it has the three-colored ones, but also a 3.5 mm input jack with an output for headphones). So, what I did was got an audio cable with two male 3.5mm ends and attached it to the input and the computer output, then I toggle between the speaker cable and headphones through the monitor output jack because it's much more convenient than moving the computer at night.

So, I'm a genius, right? Except sound quality is terrible. With the speakers it constantly sounds like it has some sound filter set to "stadium" or some shit, and with headphones there's a pretty long echo to everything and everything sounds like it was recorded in a library bathroom stall. My assumption was just that this is caused by deteriorated quality from adding more connections to the audio circuit, but I really don't know. I've looked all around with seeing if anyone fucked up the media player settings (which I use because I'm lamer than the holocaust, I know), and I can't find anything except for the her playing with the bass level and such. Am I missing something, or am I just right and therefore screwed? Is there any way to fix this?