You simply re-sample them from 48 kHz to 44.1 kHz.
I believe in Sound Forge you go to the Process menu and select re-sample.
Or you can do it by using one of the many free audio editing software such as these:
Don't apply pitch-shifting or time warp, only plain and simple re-sampling.
You won't normally loose any data re-sampling as it is merely about setting playback speed in the file header. You are only telling the player to play the exact same data, but at a lower rate. The opposite is what you're experiencing, the data played at a higher frequency resulting in higher pitch and faster playback.
However, be aware of that some program look at the system's playback rate and can actually interpolate the data and so forth. Therefor I would advice you to check other programs as well if you run into lowered quality.
If none should allow you there is a more low-level approach you can use by altering the audio file header directly (and header only) to set new playback rate. For this you can use for example
(Audacity apparently has a bug related to re-sampling - dunno if that is fixed by now.)