PA System Setup HELP!!

Hi, so I just bought a PA system over the weekend and brought it up to school (IUP) with me so I can play music at house parties, and DJ in my free time. I bought the system to start off playing music off my computer using Virtual DJ and eventually transition to playing on vinyl. So, I bought really nice speakers and an amp to put out the sound and I will get decks and a better mixer later. When I set up my system though the music sounds hollow. Some songs I will hear the background beat and no lyrics. Sometimes you will hear some lyrics but only the really loud parts and the background vocals will never come through. Put it this way, my $60 Altec Lansing computer speakers sound better than the $1200 JBL PA System that I just bought!! What did I do wrong??

Here is my setup and what I do, please help me to figure out if my problem is my lack of skill, or if it is something technical.

Equipment:
2 x JBL JRX100 - 115 Speakers :: 1000W/250W Continuous
1 x QSC GX5 Amp :: 500W / Channel
1 x Alto ZMX52 Mixer

Setup:
2 x Speakers -> Amp -> Mixer -> Computer

Speakers to Amp: Regular PA system cables

Amp to Mixer: RCA (Amp) to 1/4" Jacks (Mixer)

Mixer to Computer: 1/4" Jacks (Mixer) to 3.5mm (Computer Headphone Jack)

Here is my process: I turn on my computer and connect everything. Set all EQ to 0, Gain to 0, All levels to 0. Turn on mixer. Set all Amp levels to the lowest possible. Turn on Amp. Play music and slowly turn up levels and gain. But once they get up, the music sounds terrible. Am I setting the Gain wrong??

Please help me, I just want to know if it is me being bad at using a mixer, or if theres something wrong with the equipment.

Pictures of setup:

Front View:
http://imageshack.us/photo/my-images/703/img00132b.jpg/

JBL JRX100
http://imageshack.us/photo/my-images/39/img00142.jpg/

Back of Amp (GX5): Left: RCA Cable to Mixer. Right: Speaker Cable
http://imageshack.us/photo/my-images/542/img00152.jpg/

Mixer (Alto ZMX52): Left: 1/4" Jack to Computer. Right: 1/4" Jack to Amp.
http://imageshack.us/photo/my-images/808/img00162i.jpg/

Shitty Closeup of Mixer:
http://imageshack.us/photo/my-images/192/img00172v.jpg/

Better, but still blurry closeup of Mixer:
http://imageshack.us/photo/my-images/827/img00182y.jpg/

Cable into Computer ^.^
http://imageshack.us/photo/my-images/837/img00192q.jpg/

Some Speakers:
http://imageshack.us/photo/my-images/259/img00202y.jpg/

Hi, not 100% sure but I think the cable from th computer to the mixer is the problem. It is a stereo cable plugged into a balanced input so you will get some cancellation of various parts. Try using the line in on 2/3 or 4/5.

Allan.

"Mixer to Computer: 1/4" Jacks (Mixer) to 3.5mm (Computer Headphone Jack)"

I think this is your problem. You need a line out. Head phone jack doesn't cut it. Get a USB to line out converter box. This may help, Make sure you watch the vid;

http://www.turtlebeach.com/products/sound-cards/audio-advantage-micro-ii.aspx

Your immediate problem, as noted, is the 1/8" stereo/TRS (tip-ring-sleeve) cable from the computer plugged into the 1/4" TRS balanced channel 1 input of the mixer. I'l assume the cable is TRS on both ends and honestly that makes it something of an oddball that really doesn't have many practical applications. The computer is putting out left and right on the tip and ring and the balanced mixer input is inverting left or right and summing them. I would expect this to sound about exactly as you've described. The old McIntosh C26 had a left+right to left or mono and other possibilities switch combined with a 180 degree out of phase switch on the top and you could create some really odd sounds that would put the music on both sides or in back of you with nothing in the middle along with everything panned to center such as vocals almost eliminated.
Question - Are you out of money now that you've bought the PA? If so, you'll need an 1/8" TRS to RCA left and right cable. Plug it into the RCA inputs on the mixer and it will work. If you have let's say $150 to $200 to spend you can get an outboard DAC/ headphone amp to take USB or digital audio out of the computer and convert it to RCA analog out to feed the mixer. Fiio's are popular, I'd rather own audio-gd. This should give you a better quality of sound than the onboard sound chip in the laptop sent out through the who knows what kind of level headphone jack. If you buy the right item it may also double as a nice headphone amp for personal listening.
Your amp and speakers are solid, the mixer is really minimal and maybe unnecessary unless you want to use a mic to talk over the music. Party on..

From the described symptoms, I think it has to do with a form of "nulling". I have a "null" button on my preamp and it does the same. Basically it removes any content that is common to both channels (mono content) and only lets out the stereo content.
As suggested by others, simply fix the connection between the computer and the mixer. Use a stereo 1/8" into twin 1/4" jacks or RCAs.

This is pretty simple folks.

This mixer has a mono mic input labeled "1", and two stereo line inputs labeled "2/3" and "3/4".

All the OP needs is a 3.5mm trs to dual 1/4 ts, or to rca and adapt to 1/4. The pics are a little blurry, but it appears to have separate L&R 1/4 inch inputs for the line channels.

Yeah, what they say.
You got yourself a very expensive rear channel only surround sound system (L-R) :D

Get a good left right connection and you should be good ;)