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Making sounds
From Quake Wiki
Sound File Formats
Sounds in the original Quake are mono WAV files with a sample rate of 11kHz and a bitrate of 8. This is usually referred to as "11k 8bit mono." You can use other sample rates though.
All engines load WAVs as 16bit, unless the user sets the loadas8bit cvar (which basically exists only for running in DOS with only 8MB RAM, something that's generally not gonna happen nowadays...).
Looping Sounds
Looping sounds can be created in the Quake engine by adding a cue point at the start of the WAV file, for example with LoopAuditioner.
Engines that support Vorbis or other formats for non-music sounds also usually make the ambientsound builtin force looping (from the start of the file, when there's no explicit cue point set).