So, if I understood correctly you have a TV card with built-in sound chip and an additional USB sound card, right?
Do you see the USB sound card when you issue the command "lsusb" or "lsusb -v"?
Then, have a look at this
documentation - it's for my Linux distribution but the principles remain the same.
Third, you can have as in your case multiple sound cards installed and you access them through alsamixer with the "-c X" parameter, where the X is the sound card number, starting from 0 included, onwards. Therefore, if you have only one soundcard in your system you access it through "alsamixer -c 0" and so on.
Does this help?