About every 15-30 minutes our ejector pump goes off and just after it flushes we hear a large banging noise. It's so annoying. What can we do to prevent this banging sound? The pump is located in our unfinished basement. There isn't a bathroom down there or anything. There is 1 check valve and I can hear the water trickling into the pump. Please help.

1. Where is the trickling water coming from? Leaking toilet? Dripping faucet? Find out and repair.
2. Make sure your pipe coming from the pump is not against any ductwork, other pipes, or floor joists. Put some insulation between these items if it is that way but make sure there is no contact with ductwork or the banging will be awful.
3. Install another check valve closer to the highest elevation of the discharge pipe so there is not as much water pressure slamming the check valve shut.
4. They do make quiet close check valves but they close slower allowing more waste water to backflow back into your pit.

All of the bathrooms, shower, laundry, etc pipes flow down into the basement. There is nothing leaking or anything, just assumed that it's normal part of the waste being ejected out. I will check out the duct work because my husband did rough out the basement so we could finish it. Is there a limit to how high the 2nd check valve should be from the 1st check valve? There is also a knob on the check valve to turn off and on is this normal? Do you have suggestions on how to change a check valve and is there a particular kind to buy?

I think you should really investigate where the water is coming from that is causing your ejector pump to run so often. The plumbing from the upper levels should not be going into the sewage ejector pit. Only basement level fixtures typically go into a pit. Basement bathrooms, floor drains, etc. Something is dumping water in there to make it run so often. Ground water should not go into a sewage ejector pit.
Okay, yes the knob over the check valve is normal. They also make check valves that are just the check with fernco type (rubber) couplers on either end. That is what I would suggest for this application of putting one higher. Leave the one in that is already there also. Make sure you purchase a 2" check. As far as elevation is concerned you could put it about 12 or 18" below its highest point.
This is only to soften the blow on the check valves. It will not completely take the noise away but it will make it more quiet. If you want it to be as quiet as possible get a quiet close valve. They are spring loaded so the flapper doesn't slam shut with the backpressure. However, they do allow more waste to flow back into your pit and your pump will cycle more often.
Be aware that when you cut the pipe above the check valve there is waste water in this pipe! I would open the lower check by taking it apart below the check and use a long screwdriver to push the flapper open from the bottom so you can direct this waste into your pit or a bucket.
By the way, the knob over the check. That is only there to service the check valve. Do Not close this knob with the pump plugged in.