Things the game never tells you. Everything here works today — no downloads, no settings to change.
The game has no volume slider — its sound options are on/off switches only, with no level to set. Change it from outside the game instead.
On Windows: start the game, then right-click the speaker icon in the taskbar and choose Open volume mixer (or Settings → System → Sound → Volume mixer). The game appears in the list of apps with its own slider. Windows remembers the level, so this is a one-time job.
If you are playing full-screen, alt-tab out to reach the mixer and alt-tab back — the game keeps running.
On Steam Deck: use the volume buttons on the top edge of the Deck. Nothing else needed.
Corellia-Legacy — private Phantasy Star Online: Blue Burst