Simply use a simple recorder. I mean, Audacity on an EEE PC can do it well. Why LMMS uses a lot of resources is because the CPU works when creating the sounds, but in a sound recorder the CPU is less important as the only thing taking place is analogue to digital conversion and writing that information to disk (well that's not the end but just understand this), whereas in LMMS it is synthesizing sounds. In Audacity, import the audio you want to listen to and then on a new track record your voice.