NextPBX gives you the option to upload your own greeting in either mp3 or wav format. You can easily record a greeting using several applications for both Windows and Mac OS.
Windows offers Windows Voice Recorder as part of its operating system, and Mac OS has QuickTime Player, which will allow you to create an audio recording. Once you have made the recordings, save them in a familiar place (e.g., Documents or Desktop), and then upload the greeting by following the instructions under How do I add or change my greeting?