At this time, FFMpeg does not support looping, you will need increase the audio file length (that way you do not need to loop) or use DirectShow with the DirectSound API. You can use third-party applications such as Audacity (http://www.audacityteam.org/) to copy/loop the sound file.
To change the Codec Config to use DirectShow, open up Codec Config in E-Studio by going to Tools > Codec Config. Then select the file type under Extensions (e.g. .wav) > choose File Source (Async.) > click Choose Codec > Press X then Save.
After that, navigate to the Experiment Object Property Pages > Devices tab > Sound properties and set the Sound API to DirectSound.