It is true that removing CMOs battery overnight should have cleared CMOs. I would still try reset jumper any way.  You can find a blue jumper on board near sata ports, and another two pin without jumper in proximity, according to this video in the right lower corner of board (play time 0:50).  Usually blue jumper is on password. You move it to the empty two pins which are usually RTCRST. The Real-Time Clock Reset (RTCRST) jumper helps reset or clear the NVRAM on the computer. The The NVRAM is cleared when the jumper is set to the closed position and turning on the computer for 10 seconds.  
I bought a USB speaker from Amazon and plugged it into my Optiplex 990. When I tried to boot fault lights 1 and 2 appeared and the power button was blue later the fault lights changed to 1 and 3 and stayed that way. It was working perfectly ok until I plugged in the speaker. I removed the speaker but it stayed in the fault lights 1 and 3 all that seems to happen is the fan starts but no boot or signal to my monitor. Any tips on where to start fault finding would be appreciated             Thanks                          AlanF
Optiplex 990 won't boot