For battery-operated guitars and other stringed musical instruments, a coaxial battery-access cover cap mounts an audio phone jack that delivers the instrument's audio output. For battery access, the cover cap, which has a knurled rim for easy finger-gripping, can be removed from an attachment collar secured to the instrument with simple rotation of less than a half turn. As an option to enable the cap assembly to be installed in the instrument in place of an existing strap-peg, a strap-peg may be formed integrally on the cover cap around the jack opening. Extended embodiments can include an integral battery compartment with provision for mounting an electronic module enabling features including amplification, buffering, DSP and other effects in a self-contained system that requires only a single round opening for mounting in the instrument. The invention can be incorporated in original manufacture or as an after-market upgrade to practically any stringed instrument.