Don't have this problem. But, as far as ideas:
1. Reduce the number of airpumps by getting dual-output pumps (which may be higher quality and natually less noisy at the same time).
2. Reduce by getting 3 or 4 output gang valves, and adjust as needed.
3. See how much noise is from the air pumps and how much is from the air bubbling in the tank. You may be able to turn down the air some and quiet it down. See #2 to possible help with this.
4. More of a question: How many tanks do you have going, and do you really need that much air? Depending on the filter, I'd consider reducing it.... (Granted, if you're running all sponge filters, you can't do that.)