You should be able to open the file in Audacity, and then observe the waveform. Just scroll "to the right" to find the quiet spots.
If you want to re-edit the files to cut out the quiet spots, Audacity can do that, too.
But for trimming mp3's, I find the utility "MP3 Trimmer" to work better, because it works in "native mp3 file format", with no loss of audio quality.