There are not precise mechanics for this, so take the action that best enhances the narrative.
Looking at scale on the map of Greenest, 300' is actually a pretty large area relative to the size of the town. Also, the adventure recommends rolling for a random encounter for every 100' travelled in town. Looking at the encounter probabilities, it seems almost certain that there would be some groups of raiders within that radius.
Despite the general chaos in the town, a booming clap of thunder should be loud and distinct enough to be noticeable. How the raiders would react to it isn't clearly spelled out, however. They might be inclined to investigate, but there are various reasons that they might choose to ignore it instead, such as assuming it's the dragon's doing, being too focused on the looting they are doing, or assuming that someone else will deal with it.
As a GM who has recently run this adventure for a party with both a Druid and Bard who know Thunderwave, this is how I handled situations where it was cast.
The spell was first cast to save a group of townsfolk before the party even made it to the keep. I picked one of the more intimidating random encounter groups (with a drake) and had it approach the location of the battle. I had forewarned the party that the spell might draw attention, and they managed to spot the approaching group before it spotted them, so they ran away and hid, stumbling upon some more hiding townsfolk in the process. Responding to the thunderclap in this way added to the tension of the scene and helped maintain momentum after the battle.
The second use was against the ram at the front of the temple. The spell took out most of the encounter group and destroyed the crude ram, but it also drew the attention of the wild patrol around the temple. The party used this to their advantage, letting the patrol chase them away from the temple, leading them away long enough for those trapped within to escape. Responding to the thunderclap in this way rewarded the players by allowing their creativity to overcome a very difficult obstacle.