Within the explorer window when you have the relevant file selected, Does it say under the audio description “Audio Stream attributes cannot be determined?
if this is the case there are many variable which could be your problem.
1: When capturing make sure you that you are compressing the audio. and that you have the relevant codec installed.Uncompressed,can cause what is known as a buffer overflow without the applicapable ram.
If the audio is compressed, do an analysis of the audio stream to determine the compression of the source! then install the codec. You can probably find it by googling it!
2: Check your plug-ins, The relevant one might be missing or corrupt, If this is the case re-install the program.
3: Make sure that your version of vegas is “legit” and that it isnt some bootleg version of the pre-release beta. These are riddled with bugs and to the untrained eye looks legit but allot of the plug-ins and link (.dll) files are missing!
Another thing some cameras require their own software to capture all the data required. When this is the case video editing software that support capture can run into this problem.