Hi Rubia,
I saw your other post (directed to me) about this in the following thread:
https://discussions.apple.com/thread/3054749?answerId=25006003022#25006003022&ac_cid=tw123456#25006003
Thought it best to reply here, given that you've already had a response from Klaus.
I've just run a quick test using Audacity. Like you, I exported the voiceover from Audacity to the Desktop as an AIFF file, then dragged it into the iMovie timeline. You mentioned iMovie HD, so I ran this test in iMovie HD 6.0.3, which I haven't used for sometime now, as I now use either iMovie '11 (version 9.0.9), iMovie 2013 (version 10.0.2) or Final Cut Pro X. So I assume you are using iMovie HD 6.0.3. I'm also using Mavericks (version 10.9.2). My iMovie project was set as MPEG-4 and I imported 3 stills into the video timeline.
I had no trouble importing the audio file to iMovie (by dragging), nor moving it around in the timeline. However, after splitting the audio then attempting to delete the unrequired segment, iMovie froze. It appears that there may be an issue now with running iMovie HD 6.0.3 with Mavericks. It will be interesting to hear from others that may be running these together. Sorry I can't help further with this.
John