Very similar volume issue here
Running Volumio 2.692 on a Raspberry Pi 3B
Hifiberry AMP2 driving cheap 4ohm passive speakers
I’ve only just set this up, I installed Tuneblade on my Windows 10 Asus laptop. Tuneblade detected Volumio automatically, I started playing a Youtube video and the laptop audio comes out of the speakers. So that’s UPNP/DLNA streaming sorted! (Couple of things to report, 1. I couldn’t get Stream What You Hear SWYH to work, it seemed to detect Volumio but no audio gets streamed. That’s why I tried Tuneblade, which worked straightaway. 2. I noticed the speaker sound significantly improves massively when I put a jack in the laptop headphone socker. No idea why - might be a quirk of my Asus laptop? The jack isn’t connected to anything it’s just a 3.5mm to stereo RCA pair cable.)
I then switched to Volumio and selected a web radio station, the sound was deafening! Turning the dial down from 100 even to 1 or 2 barely made any difference. I played around with Volumio settings - Playback Options, Volume Options, Max Volume Level - reduced it from 100 to 20, press Save. Volume is now acceptable. I then stopped the web radio, changed Max Volume Level back to 100, returned to the Playback tab, reduced the Volume dial to 0, restarted playing the web radio, and gradually increased the volume and was able to bring it up to 20 or 30 without being too loud (it’s late and I don’t want to annoy the neighbours!).