Associate page w/ specific single post template?

Well one approach would be to create a generic single-media.php file and put a condition inside it which checks for the specific taxonomy. Then you would be able to include the other two based on the condition.

So, create the main template – single-media.php and the other two templates page-media.php and page-music-library.php. Then you could check for the the specific
taxonomy in this file and include the corresponding file based on the condition.

a simple example of single-media.php would be

<?php
/*
Template Name: Single Media
Template Post Type: post, media 
*/


//header and stuff


if ( has_term( $term, $taxonomy, $post ) ) {

    include '{path}/page-music-library.php';
else {
    include '{path}/page-media.php' ;
}