First thing I was aware of as a "synthesised sound" ? no idea really and I guess it depends what counts. I'd certainly have heard theremin's and claviolines etc. in the 50s, but for that matter the hammond organ is clearly a synthesised sound. (Very obviously so in some of it's 50s and 60s mainstream uses). The Dr Who theme and the other Radiophonic Workshop stuff for the BBC. The Joe Meek sound of course, but actually a whole lot of early 60s instrumental pop hits, obviously including electric guitar hits from people like The Shadows. First actual synthesiser music - again not sure. I heard Wendy Carlos' Switched On Bach in the late 60s although for me it didn't really stand out from it's 'novelty' instrumental predecessors. I think the first things I bought conscious that they used synthesisers were White Noise (the first album obviously but I'd heard the session they did for Peel first and I had the 'John Peel Presents Top Gear' LP it appeared on), that Pierre Henry/Spooky Tooth LP Ceremony and Tonto's Exploding Head Band.