Sorry in advance if you've already got this figured out, but try to raise the volume on the source. For example, an internet page that streams audio will usually have a volume/mute control (usually represented as a speaker icon.)
Also check your system volume. While recording with SoundTap it is good to get the meter readings showing in the yellow to orange area; the program doesn't offer much in accurate display, nor can you make any adjustments to recording volume. If you want to amplify your files, WavePad is a great program.