We bought our son a capture box for Xmas which has a microphone input (so he can record commentary whilst playing Minecraft and other Xbox games). We've tried two microphone's and the input sound level is extremely low (even with game sounds turned off), so I'm looking for a cheap solution. The game audio coming from the Xbox is fine, it's just the microphone input on the capture box that's low.

Is it a preamp that I need to buy? If so, can anyone recommend a cheap option (ie. under $50) or am I dreaming? I've read some stuff but thought I would check with you guys to give me some advice.

PS - I know I can use my video editing software to increase audio volume but this increases game audio/sounds as well. Also, recording commentary later via software (ie. as a voice-over) isn't going to work very well.