Wow--dedicated musician to go all the way to Rome for a music store! :-) My sister's a musician too, so I know how it is.
Yes, you can daytrip to Rome from Venice on the train, but it's pretty insane. There appears to be a Ricordi in Florence, which would be a bit better.
I did a search on music stores, Italy and found a post with the following:
L'Allegretto, via Oslavia 44, nr Piazza Mazzini
Rinascita, via Botteghe Oscure 112
Messagerie Musicali, via Del Corso 123
Ricordi, Via DelCorso 506
Ricordi, Via C. Battisti 120
Stereo Sound, via Della Minerva 9/10
Archivo Fonografico, via Le Aventino 59
Discoteca Laziale, via Mamiani 62/A
Hortus Musicus, via Le Liegi 7
Tempo Di La Musica, nr Rialto Bridge (St. Mark's side)
Store on Salizida San Lio? nr Rialto/St. Mark's axis?
Old Ricordi store 'tween St. Mark's & Accademia Bridge?
Ricordi, via Brunelleschi 8r
Music Center, Piazza Duomo 15/a
Dischi Lira, Piazza Duomo 51/r
I'd recommend scouring Venice first. If you strike out, the Eurostar will get you to Florence in about 2.5 hours. It would take 4-5 hours to reach Rome.