Splicing a shielded audio wire (microphone, headphone, or music input) is perfectly fine. In fact there are cheap intercom systems for production spamcans that use unshielded wire even for the microphone jacks

and it usually is never a problem.
The only shielded wires on an airplane that you really don't want to solder-splice are any antenna coax that carries RF, or your shielded P-leads that run from the mags to your ignition switch. And on the antenna coax, you can always use a male + female pair of BNC connectors together in lieu of a soldered splice.