As sound is the movement of particles, typically through the air, it is quite possible that wind mages would provide defense against it, especially if they were already trying to disrupt ranged attacks such as arrow volleys. If it was powerful enough, however, it would transmit through the ground, likely making people queasy and possibly making them stumble. A single blast would be fairly weak in this situation, while sustained could be crippling. Depending on the frequency, there is also the possibility of resonant waves forming, which could cause serious damage or destruction to relatively homogeneous materials such as metal, crystal, and some forms of stone, though biological objects tend to be much harder to harm that way due to the multitude of different materials.
Without protection via wind magic, such an attack would be a very potent weapon against mages, due to easily disrupting concentration, and depending on setting, possibly deafening them, making it harder to speak the proper incantations due to inability to hear their own voice.
I'd have to say it is very heavily dependent on both setting and situation due to these factors. Additionally, if it had happened before, there would likely be protection developed against it, similar to how gas masks were developed when chemical warfare was used. If it was the first time, there would likely be panic and disruption of order, with the additional difficulty of damaged hearing when trying to regain control. Depending on the views of the generals involved, they may attempt to coerce the caster into serving them, or simply attempt to kill them, with a lesser possibility being attempting to get them to remain neutral and stop interfering.
