Hello,
I downloaded the SDK for foobar2000 and I could not find the source code for the visualiser spectrum. Is it a third party plugin? Is it possible to get the source code for the spectrum. Appreciate your help.

thanks,

1st Party, but there are 3rd party spectrum plugins around. Maybe one of them have the code your after.

The SDK doesn't contain souce code to a program, just an interface to write extensions to.