Re the latex glue......I've bought 500ml pots (sold as theatrical skin or similar) off of EBay.....much cheaper than actual Copydex, but it looks, feels and ....smells just like it.
I've tried it on the new Peco Bullhead and seems to give pretty good results, the main thing being it basically is a 'permanent fix' for the permenant way.....where as pva can be released by pouring hot water over the track.
Ok.....I never use the track again....but as everything is laid on top of cork sheets, after a few moments with a spatula the track bed or cork is as good as new.
As for sound deadening, I cant see that it makes any difference, certainly not in 4mm anyway where I can hardly hear anything anyway.