Currently there's no way in the Artist > Studied at > Place relation to specify if he/she studied as a vocalist rather than as an instrumentalist. Instrument can be specified, but not vocals/vocal types.

I'd suggest to

a) either add the possibility to add the vocal types (vocal tree) as an optional attribute for this relation.

b) more simple (and probably sufficient?) would be to add "singing" as an option in the "subject" attribute for this relation (next to "musicology", "jazz", "conducting" and so on).