C-Media Audio Mixer is a component of Crystal Audio cards and embedded audio devices. If your card is a Crystal SoundFuzion or Crystal Audio device, that's what you need to use.
To access the Record Mixer, open the Playback Mixer (Volume Control) and select Options, Properties, and the Record radio button. Click OK and your Microphone, Line In, CD Player, and other audio inputs should show (if any are missing, go back into Options, Properties, and check them on in the list for Rercord mode). Check the Select box under Line In and adjust the volume as needed.
If the Crystal Audio device is an unused MB embedded feature and a different sound card is your choice for recording and playback, disable the Crystal Audio devices in Device Manager to force default selection of your other sound card for all audio functions.