48 and 96 sounds more like sampling rate, rather than bit rate. To convert analog audio signals into digital, the analog signal has to be processed. Analog sounds are constantly changing, but there is no way to represent constant change digitally, because digital is on or off, 1 or zero. So, what they do is sample the analog signal at specific intervals, analyze the signal at that instant, and save that data digitally. The more often you sample the analog signal, the better representation you get of that signal. Typical sampling rates are 48kHz or 96kHz, or about 48 (or 96) thousand samples per second. Typical bitrate for DVD is 24bits per sample. CDs are recorded at 16 bits per sample. More bits = a more accurate depiction of the analog signal, but at the expense of size of the sample.
(Of course, this is a general overview. It's a lot more complicated than that.)
Here is a Wikipedia link that has much more detail:
Sampling_%28signal_processing%29Wiki
PCM stand for Pulse-Code Modulation. PCM is just one way of sampling analog signals into digital. Here's the PCM Wiki:
PCMWiki
Hope that helps.
