I will answer in two ways.
It is easy to attach audio after detaching. The audio clip is referencing a video with audio clip in an event. Detaching it just lets you move the audio independently from the video clip. However, you can always delete the detached audio clip and go to the video clip, then open the Inspector, and turn the volume back up to 100%, and voila, your audio is back with your video clip.
However, in your case, you have audio that is out of sync, so the above will not help you. If you had Final Cut Pro, you could continue editing with the Blade tool, for one, and keep everything in sync.
In iMovie, you would have to use the SHARE menu to render a copy of you synced audio and video clips. At this point, they would be rejoined. You could then use FILE/IMPORT MOVIE to import the synced clips into an Event and continue editing from there.
The reasons for this are architectural. iMovie compiles a list of edit instructions, and does one render pass when you Share your movie. Final Cut renders in the background as you go, so it can do much more complex edits and effects.