I don't use anything but what came on the PC, and/or what a program (VS, MSP,) might install. I refuse to use codec packs, too risky.
I don't know what your origins are, but some MP3 files are real pain, because they are not true audio files, but are video files without the video. Some editors think the video is there and continue to look for that video.
One other audio problem cause would be Dolby, where you don't have the Dolby audio codec. Since Dolby is a fee-based product, if you don't have a product that includes Dolby, which there are various ones, than you can't hear it.
Are you able to see any audio information in the properties of those clips? That may give you some clue as to what codec was used.