I'm going to put a confirm or deny on this thread. Is it true that the reason for the musician trim was to visually set them apart as non-combatants. This in part because musicians were also pressed into service as stretcher bearers. The highly visible trim was in an attempt to keep them from being shot at?
It had been customary since the revolution for the Army musicians to wear distinctive uniforms... frequently with "reversed colors." In other words, where their corps' uniform was say blue with red facings... the musicians' coat might be red with blue facings, etc.
This carried over in future decades, with the musicians coats being generally red, with branch facings... and to 1851 red coats for musicians was pretty standard...
Wearing the fatigue dress, however, the musicians and rank and file would have been dressed identically.
In 1851, the Army's new uniform was theoretically intended for wear on parade and off. The musicians coat was blue like the rank and file, but with a full branch colored plastron front...
In the mid-1850s the Army dumped the branch colored facings for more modest branch colored trim round the collar and cuffs, etc.
And for the musicians dumped the full branch colored plastron front of the coat body for the branch colored tape trimming...
In 1858 the Army adopted a forage cap and fatigue blouse for all branches. This was what was worn principally in the field thereafter, showing no distinction between the rank and file and musicians...
And even in uniform, during the war, the musicians among the Volunteer troops of the Army seem to have very much made use of the common privates' uniform coats instead of the fully trimmed musicians' coats... as shown in many period photographs.