Lucent Technologies has a patent for an orthodontic audio receiver patents.google.com/patent/US6633747B1/

>In another embodiment, the switch 140 may be formed electrically and/or mechanically such that it self-powers ON only when in place within the wearer's mouth. For instance, the switch 140 may be placed such that contact and pressure from placement against a tooth holds the switch closed (and powered ON), while removal from the wearer's mouth turns the electronics OFF.