If OSS4 is installed (e.g. via some overlay), kmix is trying to compile an extra file, mixer_oss4.cpp, which has the parentheses wrong at line 289 in combination with not #defining MIXT_MUTE.
A simple patch would be to add another ) into the mix and have it compile properly.
As soon as I removed OSS4 from my system (I use alsa anyway), kmix went on to compile and install just fine.
This bug is still present in KDE 4.4.1. I will also file a bug at bugs.kde.org for this.
The upstream bug report is
This was fixed in upstream's 4.4.2: https://bugs.kde.org/show_bug.cgi?id=229910
Feel free to apply the patch from there for 4.4.1 in portage.
OK, resolving then since 4.4.2 is out...