Most sound cards allow only one analog signal to be connected to the ADC. Therefore, in order to capture audio data, the user or application must select the appropriate input source.
Some sound cards allow multiple signals to be connected to the ADC; in this case, make sure the appropriate signal is one of them. There's an API call, snd_mixer_group_write(), for controlling the mixer so that the application can set this up directly; it's described in the Mixer Architecture chapter.