FCP X clipping audio on export for no apparent reason

Have you tried trashing your preferences?

Unexplained faults like this can often be fixed by clearing out FCP X's preferences (for some reason, they get knotted up from time to time).

Download Digital Rebellion's Preference Manager (free, simple to use, and perfectly safe, both to download and use).

http://www.digitalrebellion.com/prefman/

With Preference Manager, you can backup the Prefs when FCP X (or any of the Apple Professional Applications) are working normally. Then when either of the applications are acting strangely, Trash the Preferences, then Restore from your backups (just a mouse-click).

If you trash the prefs and don't restore them, you will need to manually restore all your FCP X settings again, so it's a great idea to backup Preferences from time to time when FCP X is working well, then your backups are up to date.

Doesn't fix every problem, but it fixes a lot.

Andy

FCPX exports at extremely high bitrates, even with H.264. If your Mac lacks the processing power to play the resultant exported video smoothly, you may need to re-encode the video to a smaller, more reasonable bitrate (and the audio for H.264 should probably be MPEG-4/AAC; not WAV). If the exported H.264 video is > 18Mbits/sec, it's probably overkill. If you're going to upload the video for internet sharing, even 18Mbits/sec is overkill. 18 is as good as most consumer cameras output (well... maybe 22.5) for 1080. You can use Quicktime 7 Pro or Compressor to reconvert (transcode - although if you have to transcode, you would be better off exporting from FCPX with "Current Settings" then transcode with the alternate software.)

The way to figure out your target bitrate is to export out a 5 - 10 second clip of the highest motion video in your project (of relatively high contrasts in the scene**). Transcode at different bitrates to figure out which bitrate will give you a visually acceptable render. [For 1080, start at about ~3500kbits/sec and increase by 500kbits/sec for each subsequent test—if there's a lot of "noise" — increase by 1000's.] Five to ten seconds of video will not take long to transcode, so testing is relatively easy and doesn't take all that long. You'll get a feel for it quick after the first couple of times.

[** "even" (or low) dynamic range in a shot like a fast pan will not cause as much trouble as say, green leaves on trees in a breeze against a bright blue sky... which can cause all kinds of garbage in lower bitrate h.264 trancodes. In general: you should not have to transcode to a higher bitrate than 20Mbits/sec plus or minus 2.5Mbps (or at a bitrate higher than your video's original bitrate.) In general: you should be able to get "sterling" h.264 results for 1080 between 4 and 12Mbps. If you have to go beyond 12, then you're into an "exceptional" category. (The difference between 30fps and 60fps in h.264 is relatively negligible, and I've actually seen 720/60p encode to a smaller file than 720/30p {yeah... i couldn't believe it either—but I have seen it— so stay with the "default" framerates for the format}.)]