There seems to be a conflict between QT multimedia and mobility multimedia.

If I take the spectrum demo application, it uses just

QT += multimedia

and works on Symbian phones.

However, if I replace it with

CONFIG+=mobility
MOBILITY+=multimedia

it still compiles, but no longer works. Application runs but the mic stays silent.
FYI, on harmattan/N9 it is the inverse. The latter works, former does not.