Cubicles are not capable of isolating a person from the noise of the office around them, including conversations—nor are they designed to provide that. If your coworkers need more quiet, that's an issue they should bring up with HR.
That said, I could understand their complaint if you were talking through cubicle walls (or across the aisle) at each other. It's reasonable for people to hold conversations inside their cubicles—there's a good chance you have a phone in your cubicle, after all. It's less reasonable to hold conversations between cubicles; they may not be designed to completely isolate the worker from office noise, but they are designed to reduce the noise from your neighbors somewhat compared to a completely open office or "bullpen." You have to raise your voice a little bit to speak with someone in the next cube over, or stand up and poke your head above the divider, which also projects your voice a bit more to everyone else.
If it's really important that you have discussions during the work day with your cubicle neighbors, maybe you could get into the habit of walking into each other's cubicles ask questions or hold conversations. You could also try using the phone—as silly as it might feel to dial the extension immediately next to you—or choosing to use email or an internal chat room where appropriate.
If your neighbors complain about conversations you have at normal "indoor" volume while sitting face-to-face with someone in your own cubicle, then there isn't much you can do to resolve the problem. Explain to them the importance of these conversations to your work, and suggest they explore a privacy/noise solution with the company's HR department. It would not be unreasonable, assuming the resources are available, for their desire for quiet to be accommodated by the company.