When you enable recording for a track, the track can record from the Default Device channel specified in the Audio Hardware Preferences dialog box. This dialog box includes the ASIO Settings button, which you use to enable audio inputs connected to the computer. Submix and master tracks always receive audio from tracks within the sequence, so recording and track input options are unavailable for them.
Choose Edit > Preferences >
Audio Hardware (Windows) or Premiere Pro > Preferences >
Audio Hardware (Mac OS), and set the following options: