This may be a bit off topic for this forum, but I'm hoping for some insight - this driving me nuts.
 
One of my bands played at an area club.  They were nice enough to run an 8 track recording of the set.  I used Sonar to mix the multitrack.  Did a quick & dirty mastering of it.  Flew it over to Vegas, synced it up with the video from a camcorder that was in the audience, rendered and thought: ready for YouTube.  But wait!  I watched the video in Windows Media Player, and here's what happened:

The song starts soft and then kicks in heavy and loud after about 10 seconds.  In my mix, the intro is hitting around -10 and then it slams to 0 when the band comes in.  In windows media player, the soft intro is louder than the heavy kick in.  It's like the audio is being compressed again.  If I playback in VLC player, the audio is fine.
 
Anyone else have this experience when doing sound for a video?  If this is player dependent somehow, what will YouTube do?  I don't want to put it up until I know the audio will be right.  I'd love to hear from anyone who knows about this.
 
Seth

Keep careful attention to file info from ingest to output. Pay attention to things like file sample rate, framerate of original file vs project and output to make sure there is no spot somethig could have went awry. Another sim ple asnwer is that the output format is a codec windos media player has trouble with. Vlc has built-in codecs that media player may not have access to and thus playd a wider variety of file formats smoothly. The file may ne fine just no wmp ftiendly. Files like that will usually play back fine on youtube.

wmp is not the arbiter of either good audio or video. if vlc plays it fine, yt will too. try it.

In addition to above comments.  My local rendering is to mp4 format.
dubdisciple's comments are crucial.
Do use WMP after the fact and it does look/sound good.(Well at least decent).
The only glitches/gray area is YT does seem to reduce the overall level
on the uploads.  The YT sounds decent, but have to touch up the level.
Started using bit higher audio level in original creation.

Found the issue in Windows Media Player, and it's a sneaky little bastard.
 
Right click on the playback window -->Enhancements -->Dolby Digital Settings -->Theater
 
There are two other settings here and both of them are described as "Reduces the audio range of all Dolby Digital Content...."  So THERE IS a limiter in effect in wmp!  Might be alright for watching home videos or something.  Pure suck for music! 
 
Apparently the default setting is "Night"
 
Seth