Hi, rufustf,
I am not very familiar with iMovie 8.0.6 and so can't be much help with advising about its features re audio editing. You might consult the iMovie 8 help menu for some guidance in that area.
iMovie 8.0.6 is very antiquated and has been discontinued. It can no longer be upgraded as you have found. Further, it is not totally functional on operating systems after El Capitan, and that would include Sierra.
Since you are running Sierra you can, if you want, upgrade to Mojave OS 10.14.6 and then download iMovie 10. In iMovie 10 the video clip will display the wave forms as will the imported audio clip in the audio well below the video clip. However, iMovie 10 has a very different architecture and user interface than iMovie 8. There will be a bit of a learning curve.
Here's a link that explains the requirements for upgrading to Mojave and how to upgrade to it, if your computer age and specs allow:
https://support.apple.com/en-us/HT210190
Of course, always back up your computer before downloading another operating system.
Alternatively, you might already have iMovie 10 on your computer and thus can use that version, even if it is a bit older.
Even if you had iMovie 10 at one time on your computer, but perhaps deleted it, you could probably still update it to the highest version that Sierra can run. Post back if that's the route you want to take.
-- Rich
