I've sped up several independent audio sound effects in my timeline. They play fine on the timeline. Soon as I render it, a few of the audio clips are silent. If I open them up in the source monitor, they play fine again. Then, they will also play fine on the timeline after that. Soon as I render it again, they go silent.
For the record, it's sped up to around 1000% if that means anything.
Welcome to the forum.
What is the format and Audio CODEC in these source files? Are they MP3, or another flavor of MPEG Audio?
What happens if you convert these to PCM/WAV, prior to Import, with say Audition/Soundbooth, or the free Audacity?