Examples of codecs that Vegas cannot play are DivX, Xvid and particular Apple Codecs. So if all your settings in Vegas are correct and you are getting no audio, then you can bet that the video you are using was made with a non-standard codec. Because of this reason you will find that Software companies limit the amount of 3rd Party codecs they support, otherwise the cost of their software would become too expensive. Often these more unusual codecs have licensing agreements attached to them, which basically means they charge other companies for the right to use these special codecs in their Software or devices. Many of these codecs are invented by a particular company and are designed only for use in the devices they make. There are a huge amount of different video and audio codecs and formats in use Worldwide.
You have the Project Properties set to 5.1 Surround Sound and are rendering to normal Stereo output OR vise versa. You have the wrong Audio Device or Speakers selected inside the Vegas Preferences for "Audio Devices". You have accidentally MUTED one of the Sound Controls on the Vegas Desktop. The settings in the program and/or device you used to record your video/audio with, are not compatible with Vegas. The Video Codec of your source media is not a codec/format that Sony Vegas supports. The main causes for this problem are the following:
Vegas Movie Studio Q: Why is no Sound coming from my Videos on the Vegas Timeline? Answer: There can be a couple of different reasons why you may not be getting any sound from a Video, once you place it onto the Vegas Timeline.