That depends on the speaker model and how it implements audio detection in the auto turn off mechanism.
If it is a digitally connected speaker it may be enough playing a wavefile with silence in it.
If it is an analog connection, silence is detected, and signals below a certain level is considered "silent". In that case you need to send something just above the threshold. I'd go with a very low frequency impulse, since it can hold a lot of energy and thereby is more likely to go over the threshold.
Try a sine at 5-10 Hz with no attack and soft release.
About the ultrasonic idea: Your system is most likely not able to reproduce true ultrasonic ranges. At best you may be able to trigger it using 15 - 20 KHz, which you or your guests may hear.