I'm really surprised that nobody has come back to you yet. I'm afraid I don't have either TTS or Select, but searching the forum on your behalf for an answer I found many entries that stated that the Master Volume on a TTS model is typically controlled by CV178. The values that can be configured are 0 sound off, 1 sound low........to 4 sound maximum (which is the default). Your loco should have come with a data sheet and you should find reference to CV178 in it. Apparently, as far as I can tell, other CVs control individual sound effect volume levels.
Note the following though. In my search I also found reference to the 'Select' controller not supporting all the TTS sound functions. There may also be an issue in the ability of the 'Select' to actually write to CV178 to change the volume because the 'Select' is a really low function cut down controller version compared to Elite and e-link. Someone else who knows more about the select will need to advise on this in regard to writing and reading CV limitations.
I have just had a look at the download version of the 'Select' manual and cannot find any reference in it to reading and writing CV values. This would, unfortunately, seem to indicate that your volume is going to be fixed at the default value unless you either buy or borrow a better more functional controller.
To search the forum for yourself to find the references I found. Put TTS in the "search forums" box at the top of this page, click the green magnifying glass, wait for the results to be returned, then click the big read box tilted 'Search by Posts Content'. This will return all posts on the forum containing "TTS". You can then do similar searches using the same technique for other search terms such as "volume" "sound" "select" etc. Unfortunately, the search function is not very intelligent on this forum, it is best to search for single words. This inevitably means that commonly used words will return lots of entries.
In the meantime, maybe someone else will come back to you with a more definitive specific answer.
EDIT - once again Fishmanoz beats me to it as I was typing........he has just confirmed in a couple of sentences what I wrote in a page.