Navigate to the settings menu of the e-mail and messaging apps. There, you will find options to disable the audio notifications. Of course, to get them back again, you'd have to navigate back there and turn them back on -- assuming you remember to do that.
So a better solution is to use an automation program like Tasker, which you can program to change your profiles automatically based on time of day, day of the week, or any number of other variables. You can program it to silence alerts at night, and then unsilence them at say, 8 a.m. or whenever you want.