Diffraction limit does not allow to generate directed beams of sonic waves like it happens with a laser.
More over, sonic waves of ultra high frequency effectively absorbed by both water and air.
To make a directed weapon such as lasers, aliens must churn out generators which could form a temporarily waveguide from air/water and sonic energy must propagate within it. (radiation self focusing). To add a destructive effect, the ultrasonic waves must be modulated by infrasound frequency. This feature will help tore apart XCOM personnel and other living targets more effectively since the resonance frequencies of body parts are in infrasound range (1-20 Hz)
"Green bolts of energy" is a fiction like the "pew-pew blaster beams" in Star Wars.
Sonic pulser seems more realistic than "sonic gun". When explosion occurs, some of shockawes are pulsating within a fireball, going through it center and reflecting from its border. If the device could detect the waves and add a new pulses to amplify them, we ll get much more destructive shockwaves.
Overall, energy weapon (laser gun, plasma gun, railgun etc. ) is not a problem. Crucial problem is a lack of compact power source/power accumulator. Here comes Zrbite/Elerium