Front speaker/headphone jacks acting weird

I've been using my Dell Inspiron 1420 since August last year, and so far the sound never gave me any problems.

I use a pair of relatively cheap speakers and a set of good quality Phillips headphones every so often. Since I don't really pay too much attention when listening while doing assignments and have minor hearing problems as well, it took me a few days at first to realise that something was off with the sound coming from the speakers. The right speaker and the subwoofer were working fine, but the left speaker only produced a faint buzzing noise.

Initially I thought that it was a speaker problem, and that something was wrong with the left speaker; I noticed that every so often the left and right speakers would both work perfectly fine in cold ambient temperatures (usually after the air-con was on for about 4-5 hours and the room is freezing), and so thought it was a contact problem. Then I used the pair of Philips headphones I knew worked perfectly well, and found the same problem. It's always the left side which either produces nothing but a faint buzz, or the music comes out much softer than the right. This happens on both headphone jacks.

If I plug in my headphones half-way into the jack, the sound evens out and all sounds pretty normal--however that fix doesn't really work for my speakers as I get a loud buzzing sound and then either the left speaker or right speaker alternately produce no sound, or both produce very faint music with a pronounced buzzing that about covers the music.

Any suggestions as to what might be wrong and ideas to fix it? Or should I just cart the whole thing off back to Dell for them to look over?