Looks like AOPA measure it in their flight tests:
Measured cockpit noise (dBA scale) in cruise at 11,500 feet msl was 88 between the front seats. This is about 4 dB less than most single-engine airplanes.
Diamond Aircraft DA42 Twin Star - AOPA
Questions you would want to ask are; was the dB meter calibrated, what frequencies did it measure, was the same meter used for each test, which way was it pointing and was it rigidly mounted or handheld.
Apparent noise level to a person may differ from noise meter readings.
Quite a lot on Cirrus cabin noise here too:
Cirrus SR20 (and a bit about the SR22)
Bonanza here:
http://www.aopa.org/News-and-Video/A...ophonous-Cabin
DA40 here:
http://philip.greenspun.com/flying/plastic-airplanes