Hello eider2018,
Thanks for that info and for choosing the Apple Support Communities. If we understand correctly, you are unable to hear audio from a video that was stored in iCloud on your Mac in iMovie and possibly QuickTime. So we can help isolate this, we recommend importing this video to the Photos app and see if it plays properly there: Overview of importing photos and videos into Photos on Mac - Apple Support
Another way to isolate this is to play the video on the iCloud.com website and see if you get the same issue: View your photos and videos on iCloud.com - Apple Support
If you are still having this issue, we recommend following these steps that Apple recommends for this situation:
"Older or specialized audio, video, or image formats might not be supported on your device, or might require different software.
Apple apps such as Photos, iMovie, Safari, Keynote, and QuickTime Player work with many media formats. Some apps prefer specific formats, but most formats work in most apps without additional software. Older or specialized formats might not work as expected if your device, its operating system (macOS, iOS, or iPadOS), or the app you're using to view or play the file doesn't support the file's format.
Install all available software updates for your Mac and software updates for your iPhone or iPad. After updating, if you still don't have an app that supports the media format used by the file, search the internet or App Store for an app that does. You can search by the name of the format, or by the filename extension, which is the letters shown at the end of the file's name, such as .avi, .wmv, or .mkv.
On Mac, you might be able to identify the format by selecting the file in the Finder, then pressing Command-I to open an Info window. The format might appear after the label Kind, such as ”Kind: Matroska Video File.”
And those steps are from here: If a media file doesn't open or play on your Mac, iPhone, or iPad - Apple Support
Cheers!