Hi All
In all honesty, safety valves are loud. And the sounds will reflect this. It's possible in future that we might be able to add a volume control for this, but at the current time this isn't possible.
It should be noted that the enginespec should be relatively controllable, using the available controls in Trainz, to manage your boiler pressure. If you have them blowing off all of the time, then you have too much fire for the work you are doing. Additionally, your enginespec should be set so that the safety valves take at least a few seconds to bring the pressure back down. If they release too much too quickly (ie the flow rate), you'll cause them to repeatedly blow off for a short time, and this will keep happening till eventually the fire temperature drops. If the flow rate is too low, you'll get what is called 'accumulation' where the boiler pressure will continue to rise (this is a VERY valid problem on steam locomotives!).
It should be expected that, if you're working hard and then suddenly back off, your boiler pressure will rise and you'll blow off for a short time. But with suitable fire management, blower management, and water management you will only have this for a short time. But this is only if the enginespec's configuration is setup right (and this does require a LOT of testing to get just right).
I have, however, entered a feature request for a volume control to see if this can be done in future. But I can't say when or if this may actually be implemented.
One option for the more adventurous among us, in the interim, is that the sound is contained in asset <kuid:570470:100003> Sounds, which can be opened for edit, and 'obsoleted' (you cannot directly submit the asset). Note we cannot provide technical support for this, but it should be possible for those with asset editing experience to alter or replace the safety valve sound and creating an obsoleting version of the asset. It would, however, prevent multiplayer sessions from functioning...
Regards