low-level audio signal at 15m distance
Really can't recommend trying to make a poor headset microphone work over 15m, even with really good cable. The amount of noise you pick up stands in no relation to the current produced by such a microphone.

Since 15m (and the earlier 10m, but it makes no sense to work with two 
specifications, if one is harder) is too long for USB, you'll have to have box at close to the PC that either enhances 


the robustness of the digital signal, before the conversion to/from analog happens at the headset end, or
the robustness of the analog signal going to the headset and, a box at the headset end increasing the robustness of the analog signal going to the PC side.


First of all, in professional (stage) audio, 15m is "not much", and is pretty normally driven with balanced audio signals – but that means your microphone will have to be balanced, and typically active (typically, an electret using the phantom voltage from the receiver) and typically more expensive. So, cost-wise, this isn't that attractive.

You can usually re-drive USB 1 and USB 2 signals pretty reliably; basically, have a USB hub after a couple of meters, then another one a couple of meters later and so on. In fact, there's even "active" USB extension cords that pretty much do that. So, that's your easiest solution. Attach a USB sound device at the end. Done; nothing to "design" on your part.