A SND (SouND) file is a digital sound file format that was created by Apple Inc..
Data stored in such files are commands for the Macintosh Sound Manager including wavetable instruments and sound samples. Thus it is some kind of improved midi format. As well it can serve as a pure audio file format if only one command and one sound sample is stored in it.
Many other programs use audio files with the ending .snd.
Most likely it can be confused with the au file format used in operation systems from sun which sometimes also uses the extension .snd. Both data types differ in its header bytes. Sun uses the first four bytes of the file containing the magic number 0x2e736e64, while apple uses the first two bytes as a 16-bit integer contain the number 1 or 2.
The manufactorer of electronic music instruments AKAI had an audio file format with the extension .snd. The first byte contains the number 1 and the second the number 4.