On a search results screen, use the Location limiter (from the choices down the left side of the screen) and choose "Music and Media."
In your search boxes, change Any Field to Location and enter "Music and Media" to limit search to library's AV Collection.
Location contains Music and Media
AND any field contains Beethoven
You can try searching for the opus/catalog number (e.g. BWV 22) of an individual piece. Most, but not all, library records list these numbers in the record for that recording or score. If searching by the catalog number does not retrieve the desired recording, try searching by the composer and several words from the title.
To find opus/catalog numbers, one authoritative source is the New Grove Dictionary of Music and Musicians (part of Oxford Music Online), searching for the composer name and works.
Example Searching in OneSeach for recordings of Johann Sebastian Bach
With and without catalog (BWV) number
With a BWV Number:
The OneSearch record for Gustav Leonhardt's harpsichord recording for Bach's Goldberg Variations does not list any BWV or titles of individual pieces. It will only be retrieved by searching for general terms like Bach and harpsichord.
Without a BVW Number:
The OneSearch record for Chicago Symphony Orchestra's recording of Bach's St. Matthew Passion oratorio does not list any BWV numbers (and in addition it only lists the title in German - Matthäus-Passion). To find this item in OneSearch, you need to search for terms like Bach, Chicago Symphony, Solti, Kiri Te Kanawa, and Matthäus-Passion.