When you say they "leave town", do you mean to go to work during the day, or for several days on vacation? Because dogs cannot be left alone inside for days on end with nobody to care for them - that's grounds for a complaint to animal control because they may not be taken care of properly. Unless they have a pet sitter coming over several times a day or staying there, they are possibly guilty of neglect.
If that is not the case, and the dogs are not outside barking, and you are not in an apartment complex, and it's during the day, so I don't know really what you could do, unless like others have said, it's after a certain time of night. If they were outside (where the barking would be louder and potentially bothering more neighbors) or you were in apartment, you could file a complaint, but otherwise I'm not sure. If the dogs are left alone that much all day long, they are probably barking because they are bored. Little dogs do need exercise - it may not take as much to wear them out as a bigger dog but they still benefit from it. The owners may not even be aware of the problem. You could mention it to them but in a way that is not confrontational, but more concerned - as in, "Are your dogs okay during the day while you are gone? Because I can hear them barking non-stop in your house while I am in my house." And see what they say. Sounds like dropping them off at a doggy day-care (with the option of overnight boarding) might be the best solution!