the sound is muted by default in the greeter

Originally created by @goupille on #17666 (Redmine)

we received a report from a user asking that the volume in the welcome screen to be set higher, currently it is not possible to use orca easily if tails is started in VM, for example, the volume keys may not work and the user would have to find the volume before the screen reader actually read the screen…

Parent Task: #14522