Cannot play files in document folder

The native audio plugin is working really well for files provided in the build, but we are downloading files from the cloud and storing them in the document folder of the app (e.g. /var/mobile/Containers/Data/Application/5CF2D476-0B24-4F2C-A331-AEECDF390071/Documents/ in my case), which is where Capacitor filesystem saves them. We would like to play them from there.

However when I try this, and I set the path to, for example "var/mobile/Containers/Data/Application/5CF2D476-0B24-4F2C-A331-AEECDF390071/Documents/1630516106806.m4a" then I get an 'Error: Asset Path is missing' error message.

Is there a way around this? I tried "/var..." and "file://var..." as the path to no avail.

Any help much appreciated.