They are not as thorough as listing them one at a time but are a great time saver. There is also software available that you can run on your computer and accomplish the same task. Listing disappear from some search sites after a while, so listings should be done very couple of months. Also obtain listings on as many audio and music-related sites as possible. Obtain listings on sites of your suppliers and customers and of any related companies.
Many associations provide listings for their members.
There are generally local business listings available in your area where you can list your business by category. There are no shortcuts for these, but they will bring the most qualified traffic to your site. The search can go on forever, so be patient. Remember that it is quality and not quantity that you are after. You will be hard pressed to deal with thousands of consumers looking for low-end stereo equipment if you don't target your efforts.