Environmental sounds have been frequently used in ERP studies. You might start be looking at some of these papers and seeing if there is standard canon of environmental stimuli. One such study is Cycowicz, Y. M. & Friedman, D. (1997). "A developmental study of the effect of temporal order on the ERPs elicited by novel environmental sounds," Electroen. Clin. Neuro., 103(2), 304318.