These systems appear to be using the AMI BIOS chips
The beep codes for 6 and 7 beeps are as follows
6 Beeps
Six beeps means that there has been an 8042 Gate A20 test error. This beep code is usually caused by an expansion card that has failed or the motherboard that is no longer working.
7 Beeps
Seven beeps indicates a general exception error. This AMI beep code could be caused by an expansion card problem, a motherboard hardware issue, or a damaged CPU. Replacing the faulty hardware usually fixes the cause of this beep code.
This appears to indicate that the motherboards are the issue on both beep codes, I don't think this model comes with an expansion/riser card.
You could always swap the CPU from a working machine to make sure the 7 beep codes is not caused by the CPU.
But the most likely thing is that the motherboards have simply come to the end of thier life and the machines have seen better days
From what I can see these systems came out around 2006 and this would not be a shock that they have come to the end of their life, in fact I would go as far to say that getting 5-6 years out of a work computer is not bad
