Trademark laws exist to help you protect things that are important to your business. Often they identify that business to your potential customers. That's why you trademark a logo, for instance. You want customers to instantly connect it with your brand, and you do not want another company to use it and benefit from your reputation.
That's the same reason you may want to trademark a sound. It can also be strongly connected to your brand. While the most common example is a short jingle that plays during a radio commercial or a television commercial, it could be even simpler than that. Many companies have trademarked short snippets of sound that they have created.
For instance, NBC Entertainment holds a trademark for a three-tone chime. They can use it along with their logo in a television or internet ad to instantly tell people who owns that content. It's so simple that most people couldn't hum it on their own, but they'd know exactly what company it was connected to if they heard it.
This example helps to show the tremendous power of consistent branding. It does not have to be complex. It does not have to be expensive. But, when it is repeated often enough for customers to connect it to the brand, instinctively, then it can be very powerful.
As a company owner, you put a lot of time, money and energy into developing your branding. You do not want to put that at risk. Make sure you understand what legal options you have to protect the logos, colors, sounds and other identifying information you use.