Please explain what your signal chain is first so we can have a frame of reference for where to start to help you man! I assume your sound card is properly installed. I will also assume you have the mic connected to your mixer. When you talk into the mixer, i assume you hear sound coming from speakers powered either by an external amp or internal amp, or powered mixer amp. So, now here is where you need to add some info. You are connected from the mixer line out of some kind to your sound card input, correct? Your sound card should have software which controls the inputs and outputs, correct? And I assume this is where you aren't showing any signal to the sound card?
