trademark infringement law, is it infringement if a trade name just sounds like another one

The key would be "confusion" and "dilution." Is it likely that someone would think your product is linked to the other product? Will your use of the similar name dilute the other's brand in the market place? The real question to ask yourself is this: "Is your proposed name so vital to your business' success that you are willing to potentially infringe on someone's trademark or to risk litigation even if you have a defense to an infringement action?" Generally speaking, if the name is that important to your business plan it usually means you are hoping there is confusion or gain by association with the prior makr. If it is not vital, then pick somethiing totally original and avoid legal fees and hassle.

The fact that you're seeking online legal advice before making a branding decision about your company is a step in the right direction. But it's a baby step.

A company's name and its trademarks to a very large extent create and carry the reputation of the company -- i.e., the goodwill, the marketing buzz, the brand identifier, whatever you want to call it. Making a key branding decision without including a trademark lawyer in the process is extraordinarily short sighted. And I would argue takes the decision out of the protection provided by the business judgment rule (which protects officers and directors from liability for making even real bad decisions -- but only if they've taken reasonable steps to educate themselves about the matter). Lecture over.

As Oscar notes, a person cannot use a trademark that will likely confuse consumers in the marketplace in light of already existing trademarks -- and that includes a prohibition against using the sound alike mark that you describe. But there's much more to it than that and you should spend at least a few hours with a trademark lawyer to more fully address your situation and to discuss more generally your branding decision.

I agree with the comments posted above.

I would add, however, that you said "the section that we would like to register in is the same as the other company." I take it to mean that the goods you propose to market are listed in the same International Class as the goods for which the original trademark was registered. You should know that this fact is not dispositive of whether or not there would be a likelihood of confusion. In certain situations, it is possible to register a trademark for one kind of goods in a class while a similar-sounding trademark is registered for another kind of goods in the same class. Conversely, the mere fact that your goods and their goods are classified in different classes is not dispositive that there ISN'T a likelihood of confusion. What happens is that a Trademark Examining Attorney would consider the specific goods listed in the registration and compare them with the specific goods listed in your application and surmise whether or not at least some prospective customers would be likely to think that their source or sponsorship is the same as yours. If so, the two trademarks would not be allowed to coexist on the federal register of trademarks.