I think this is an interesting question. So, I've noticed that certain sounds in some songs can put me into a rather wistful mood for a time I never even lived in. It seems to me that specific elements to some songs bring this out more than others. I will give a few examples.
The Fade In/Fade Out
I find songs that have a fade-out in them can be very nostalgic, it's like the sounds of a bygone era slowly fading to nothing, much like they did in our culture.
Examples-
Echoes
I think the echo is also a very nostalgic sound, I can't quite pin down why but it makes me rather wistful. It sounds almost ethereal, like a ghost of the past.
Examples-
Wavey/Dreamlike
Songs that have synth tunes in them that seem to wave back and forth in harmony with some distortions thrown in are also pretty nostalgic sounding to me, it makes the past seem magical.
Anyway, these are opinions. What do YOU think makes a sound, nostalgic?