You cannot import a video that contains audio into flash as the audio and video content will go out of sync after 15-25 seconds in my experience.
The simplest way to do this is to play the flv file as an external item, (sorry don't know how to do this using flash CS5 as I only have CS4, maybe someone else can help) but I normally do this via an xml file and use the flash 'stage' as a holding area for the content if it is a slide-show, (or use the html5 video tag if it is just a single flv).
If you could tell us exactly what it is you are trying to do, then it may be possible to help further.
The biggest issue with out of sync audio/video when "embedding" an flv into the main timeline is that the frame rate of the Flash doc is not set to exactly the same frame rate as the video. So if the video was shot at 29.97 frames per second, you need to set the Flash doc to exactly that, 29.97. And unless there are some Internet download connection speed issues, it pretty much keeps the audio/video synced.
It only makes sense that playing a 29.97 video file in a 15 frames per seond Flash doc will slow the video display way down, since one frame of video equals one frame in Flash... but the audio part just plays at normal speed... otherwise it would be way distorted, like playing a tape/record in slow motion.
Here's a short sample video embedded directly into the timeline in a Flash doc set to 29.97 frames per sec:
That being said, I would still not recommend embedding video into the main timeline except under special circumstances. Use the FLVPlayback component or NetStream to play an external video.
Whilst I do agree with what adminiastratot has said, the other thing to be aware of when using this technique is that doing so will cut the swfs usable time to slightly more than 9mins and any animations etc that you already have will also be affected by this, so it cannot be used with any swf longer that 9mins. There are a few work arounds that I have tried, but none that I can recommend.
But the technique does solve the audio sync problem.
Again, it's a specialized solution that you may want to use only for a few specific cases.... just another tool to add to your toolbox!
And while I don't use it very often, when I do, I usually embed the video into it's own .swf and then load that .swf into a main .swf as needed.
And with AS3, you can give the video .swf it's own frame rate... if that's of any help.