On most modern Macintosh computers the audio card goes in to sleep mode if no sound is played for a period of X seconds. This probably saves energy, and that is probably why the feature is implemented. Apple has not yet implemented any feature to turn the automatic audio card sleep mode off since they probably do not think it would ever cause any problems.
The automatic audio card sleeping can however cause problems for some users, and this is where Audio Awake comes in to play.
If you are experiencing audio issues like humming, popping, or other types of noice with your Macs audio output it could be that you are exposed to this problem.

1. It seems that, when the Macs audio card goes in to sleep mode, the audio output cable is no longer grounded (or grounded in a different way than when the audio card is active). If you have connected your Mac to an audio amplifier, like me, you may hear a disturbing humming sound in the speakers as soon as the Macs audio card goes in to sleep mode.
2. When the Mac audio card is in sleep mode there is a slight, but noticable, delay before a sound is played, since it takes a little time to wake the audio card. If instant audio playback is desired, it may be pretty irritating with this delay whenever the audio card has gone to sleep.
3. Some people have reported disturbing popping and other types of noice in their speakers whenever the audio card goes in and/or out of sleep mode.

Audio Awake is a simple Mac application that prevents your Macs audio card from going in to sleep mode while the Audoi Awake application is running. Audio Awake does not change your system in any way, it simply plays an extremely short and completely silent sound at a set interval (every 4-59 seconds) to prevent the audio card from sleeping. Audio Awake uses hardly any computer resources at all, and is designed to run, out of your way, in the background. Audio Awake can be used as a login item.

Audio Awake is provided as is, free of charge.
To use it just download and start the application.

Download Audio Awake 1.1 for Macintosh HERE (3.8 MB).

Audio Awake works on Intel Macs with Mac OS X 10.7.5 or later.

• (2016-11-11) Version 1.1 released. Added functionality to control the audio card wake up frequency.
• (2011-11-07) Version 1.0 released.