There has been many attempts at creating shared API's to cross multimedia framework boundaries like UHAPI and OpenML. So far none of these has seen wide adoption partly cause they tend to stumble on the problem of ending up only supporting the lowest common denominator and thus the market tend to eschew them for the using the media frameworks directly.
For KDE 4 Phonon is set to provide multimedia functionality without depending on one particular implementation. In the KDE 4 codebase Phonon already has seen wide adoption. Phonon's API is limited in comparison to GStreamer or NMM, but that is because it already implements configuration and policy.
In terms of configuration things have been improving a lot over the last year. Low level infrastructure like HAL has created a baseline set of configuration data for detected devices and in GNOME there is a growing set of shared GConf keys to control various multimedia output. These shared GConf keys are taken advantage of by GStreamer through some special GStreamer GConf elements which let GStreamer applications take advantage of these settings without requiring the application to be a 'GNOME' application.
Advanced multimedia support under linux is still rather new, and there is still few applications trying to take advantage of various kinds of multimedia hardware. Thus applications trying to take advantage of various hardware tend to run into a lot of driver issues. Due to new applications using for instance GStreamer starting to pop up the driver/system API's are getting more exposure and bugfixing, but still using Firewire, V4L1|2, USB, DVB and so on can feel a bit like hit and miss under linux.