I would think manually training your Echo would be easier than waiting for it to learn by amassing a large number of beeps (and trying to infer patterns from timestamps and your presence/absence). I would rather say something like "Alexa, listen to this notification sound, save it as 'oven done'." Then in future to be able to just say "Alexa, do VERB when 'oven done'".

Alternatively, Alexa could ask "I just heard a ding, was that your dishwasher?" and maybe compare against a database of known devices.