Generate simple sinusoidal sounds and plot signal/fft
A lot of things here are already good, like:

fs, duration and t being members on SoundStimuli
The choice of which things to initialize in the first __init__ (mostly)

Things that could be improved:

SoundStimuli is arguably SoundStimulus
self.signal should not be an optional member variable; the base class should be made effectively abstract and the signal should be returned from a getter or property whose base implementation raises NotImplementedError.
In play, the absence of a signal should be fatal, so you should naively access self.signal, and let any exceptions fall through to the caller
After p.open, wrap the rest in a try, and put the close() in a finally
convert2float is not a great idea - it replaces a member with a different version of the same member. That means that before this is called, the instance is by definition in an invalid state, which should be avoided.