The stereo in my 2003 Altima looks as if it's working(the light are on, it changes channels, and the clock works) however, there's no sound

The woofer is on a different fuse than the rest of the radio. There are fuse boxes in several locations on the Altima, check the box under the steering wheel. Look at this link:
http://www.carcarekiosk.com/video/2002_Nissan_Maxima_GLE_3.5L_V6/fuse_interior/replace

The channels r blown, u need a new radio

You should check with your dealer for how to reset the radio after someone has tampered with it. This has happened to me and it appears someone had tried to remove the Bose stereo...thorough detailing!

if everything works when you jiggle the ignition... then its the ign switch try to see if wire harness plug is loose... if not have ign. switch tested then replaced..

sounds simple but check there is live at the bulb with a multimeter or 12v test light. should show round 12v. if not then it will be a wiring fault/broken circuit or bad earth. if there is then check the bulb connections and change try changing bulb even if you feel it may look ok. looks can be decieving

if the radio and DVD work, and only one channel comes on when you plug in your iPhone (i assume used for music), you may have a mono plug
(the tip looks like two pieces of metal, separated by plastic), going into a "stereo" jack (the stereo plug going into this should look like three pieces of metal, separated by two plastic).
for the earpiece with mike, used on a cell, that plug would look like four pieces of metal.

Is it a standard version or Bose version? If it's a standard version then either the plug in the back came loose or the built in amp in the stereo is dying. if it's a Bose, then there should be a pre-amp some where under or behind the dash or possibly under or behind the rear seat/ trunk. In that scenario it's possible that the amp has gone bad. Another possibility regardless what version system is installed would be if a wire cracked and is showing somehow, grounding out causing the speakers to go out. Bad ground. maybe a loose plug behind the stereo. Also has the stereo every been replaced. Is it factory stereo and speakers.

is the radio connected to a relay under the hood?

lol...My 02 Altima Original Radio....wouldnt read ..or skipped on Cd,s in the morning ......in the evening it played fine.....I replaced it with one that plays mp3,s......and very happy now :) 6 years is the norm for a radio...

Just get to your mechanic till its not too late