It sounds like 什么(shenme) to me, which means what .

Hi Brady, I'm not a native, either, so please take what I have to say with a grain of salt. I would definitely agree with Edi that you first have to make sure they're actually speaking Mandarin when they say "sa ma" because even if they are able to speak Mandarin, they may not speak it among themselves. They may be speaking their regional version of Mandarin that even other native speakers find hard to understand or they may be speaking one of the many languages of China that's not even Mandarin. For example, I go to LA Chinatown at least twice a year and I can tell you that while almost all shop owners and restaurant workers can speak Mandarin but I hear Cantonese and Toisanese(?) around me. Anyway, another thing I notice is that in America, I would say, chances are, about half if not more than half of Mandarin speakers you bump into will have some sort of a southern accent than a northern one. So, to me, "sa ma" sounds like you may have heard "什么? (what?)", "是吗? (is that right?)" or "是吧? (right?)" with a southern accent... Let us know what you find out! :)

Obscure hearings do not make sense, the sound is hard to describe just using "Sa Ma", for you are not a native speaker, we do not have the same pronunciation. By the way, China is a very large country, it has many different dialects, and you didn't say where you were working in. I suggest you to record the sounds by your cellphone, no matter they are spoken by the guests or you, just make sure the pronunciation is absolutely correct, and then, play the record to your friends, ask them what it means.