Can an eight ohm speaker produce ultrasonic sound?
Most standard speakers such as those used in your stereo or the little speaker inside your desktop computer will NOT work well at ultrasonic frequencies.

The speaker impedance has nothing at all to do with the frequency response.  What matters is the size and mass of the speaker cone and the inductance of the speaker coil.

Piezo transducers are an entirely different story.  One of my favourite ultrasonic transducers is the piezo tweeters that used to be commonly available in the '70s and '80s for use in stage monitor speakers and such.

There are several reasons why piezo transducers work well at ultrasonic frequencies.

1) low diaphragm mass

2) impedance that goes DOWN as the frequency increases