I am looking for a sound meter app so I can measure the volume produced by the speaks.
I checked F-Droid but didn’t find anything. When I check Auroa I find some, but they all want to view network connections.
I have a preference for FOSS software, but will use closed source with reasonable permissions.
Aurora (or just Google Play) volume measuring apps usually want network permission for downloading ads, i guess. It isn’t necessary for anything else, unless the app wants to upload your measurements somewhere. You can use any of these apps safely if right after downloading you forbid them from using network (or any undesirable permission for that matter).
Just go to the app’s page in your phone’s settings, where all installed apps are listed, click on “Permissions” or “Network connections” and choose “forbid” or uncheck all checkboxes regarding network.
I also checked F-Droid and couldn’t find a suitable sound measuring app. There are some but they all are not fit for your use case, except for one, Noise Capture. But even this one wants network permission! 
But for a good cause. You can revoke any permission from this one just like from any other app.
You can check out these FOSS apps, but i doubt they’ll suit you: Audio Spectrum Analyzer, AudioMeter, OpeNoise (also available in IzzyOnDroid repo for F-Droid).
PlainApp: File & Web Access (Manage your phone on the web! Access files, contacts, videos, music & more.)
I’m not entirely sure I understand what the app will be used for, but Spectroid on the play store sounds like it will work: https://play.google.com/store/apps/details?id=org.intoorbit.spectrum
It requires literally only the mic permission (and sensors if you’re on GOS) and will give audio levels in DB for any frequency it hears.
