Essentially, any sound going into the laptop, from an external microphone or from the internal one itself and even from direct input (i.e., plugging a (music) keyboard directly into the laptop) will not record properly: any sustained sounds decay in volume much faster than they should. The volume, after about one second of recording at the correct level, immediately drops off to less than a quarter of what the actual volume is.
Needless to say, this makes recording music infuriatingly impossible.
Any help is much, much appreciated.
I'm using an Acer Aspire 6920, running Vista.