Haven't looked at SoundForge; Audacity I know supports this well.
I use Wavosaur and zoom in on the samples with a magnification on the sample display to distinguish from absolute silence; with certain N64 songs (while few, of course) absolute silence is between each loop of an infinitely looping song.
So I either trim the silence off both ends of the infinitely looping sequence for the "original length" (a confusing term which instead I will arbitrarily use in reference to the length in non-silent samples of one complete loop of a sequence) and set Winamp to play it with loop, or I'll just cut the number of samples down to an exact number of minutes (like 1:00 to 5:00 if possible), then doing 10 * (samples per second frequency) and select that many samples at the end of the WAV to apply one of various fade-out algorithms.
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