This is most likely some inept oversight on my part, yet it’s always best to have more than one set of eyes when analyzing a problem.
EDIT:
You can always setup your own "conversion-settings" and point the software to "lame.exe" (LAME Binaries can be downloaded from RareWares (http://www.rarewares.org/)).
Turns out that the zip from sourgeforce didn’t even contain the lame.exe file, whereas the Lame download from RareWares was indeed the correct one. They were both the same version (lame 3.99.5), too.
Thanks, everything works perfectly now. Literally spent hours chasing a wild goose.