Furthermore how long have you owned the Play: 1 or Sonos One as many get them confused? Both have been discontinued for quite some time. So when did you discover the speakers aren’t playing loud enough?
There are no standards with respect to Volume percentage and “loud”. Further, we humans have been conditioned to equate distorted with “loud”. As level is increased beyond a unit’s design limit, distortion increases rapidly and the unit gets “loud”. SONOS units are designed not to enter into this high distortion mode and will therefore never become “loud”.
In my college apartment we could run at levels that made verbal communication very difficult, but we were not anywhere near the distortion threshold. Yet guests would approach our ears, hands cupped around their mouth, asking us to “turn it up” because it did not seem “loud” yet. This same crew in another apartment, running a cheap, struggling compact unit, would never ask “turn it up”, conversation was easy and it was “loud”.