Bug 34361 (https://bugs.webkit.org/show_bug.cgi?id=34631) added support for the QtMultimedia framework in QtWebKit. At that time, QtMultimedia had been integrated to Qt 4.7. So the code uses preprosessor code such as #if QT_VERSION < 0x040700 to decide whether to use Phonon or QtMultimedia.
However, the high-level playback functionality of Qt Multimedia will be removed from the final version Qt 4.7. So this code will not work (or compile) on Qt 4.7 anymore.
Committed r58390: <http://trac.webkit.org/changeset/58390>
(In reply to comment #1)
> Committed r58390: <http://trac.webkit.org/changeset/58390>
Cherry-picked into qtwebkit-2.0 with commit 3f78c44aa21a5140996abcef10ae12a5df27dbd0.
Bug 38612 includes a follow-up fix.
Both fixes made it into Qt 4.7 last week :)