For now, the 8.0 community repositories are working well with 8.5, except you will need to pin the mlt and libmlt packages that come with 8.5.
I have noticed that KDE 4's Kmix is often missing sliders after installation, so go into the mixer's Settings-Configure Channels, then enable them all, and enable the slider labels, too. Then play with them to see if that helps.
VLC and SMPlayer also seem to have better output for me than KMPlayer, especially after I put the volume multiplier in SMPlayer up to 900%....
I think KMPlayer is using Phonon or xine, which does not seem to put out a lot of sound by default.