Ok, mine does have a speaker grill below the cp, no speaker there, though. I'm assuming the one below the marquee is an 8ohm; though I could be wrong. Not sure why it sounds crappy. I just replaced one in my Battlezone cabaret that had the same crappy sound. The speaker was really old and brittle. I'm thinking this might be the same problem.
First of all, where can I get new ones? Secondly, how do I go about wiring in a second speaker(below the cp)?
Here's what I would do:
First, check your top speaker to make sure it's in decent condition, hooked up with the proper polarity (white wire to positive terminal) and that it's 8 ohm (impedance should be stamped on the magnet). You'll need to remove the marquee, and then pull out the light/speaker assembly. See page 3-6 in the Gravitar manual.
If you don't have a manual and schematics, you can download them from
http://arcarc.xmission.com/PDF_Arcade_Atari_Kee/Gravitar/
Next, if you haven't done so already, rebuild your A/R-II board. The big electrolytic capacitors in the audio amp circuitry are far past their lifetime. I recommend Bob Roberts' kit - "Atari A/R II-02 to -06 Repair Kit (Caps+Q2/3/8/9+R30) $12.00" at
http://www.homearcade.org/BBBB/parts.html
You'll also need some TO-220 and TO-3 mica insulators and some heat sink compound.
(This assumes that you have appropriate equipment and are comfortable with desoldering and soldering components.)
You may want to also order two of "TDA 2002 Atari A/R bds & others sound $3.00" just in case your problem stems from bad audio amps.
After the rebuild, be sure to check and adjust for a good +5V on the Gravitar pcb.
If you're doing that much, you might as well bring the rest of your power system up to spec. Get "Atari AC Power Supply Repair Kit + 8' Line Cord $27.00". That gets you the parts to fix up your transformer block (filter cap, bridge rectifier, fuse block, fuses, power cord).
Next, it sounds like you have an empty hole where you should have a lower speaker. Are there screw holes where it looks like a speaker was once mounted? If so, look to see if the speaker wires are dangling loose around there somewhere (a white wire and a brown wire), or, check the nearby wiring bundles to see if the wires are there, but got snipped off.
If the wires are there, you'd just need to screw a speaker in place and connect it. If the wires aren't there, then you'd have to splice some into the harness - easiest spot would be at the 12-pin molex connector going to the service panel.
For replacement speakers, Bob sells those too:
"8 ohm 15 watt shielded 6X9 speaker $10.00"
Note - you may find this restoration thread of mine helpful:
http://forums.arcade-museum.com/showthread.php?t=104226