as mentioned in title, I want to be sure that a sound I found on youtube (2 seconds long) is copyrighted (and also, if I will modify it (in audacity or something) will it still break copyright laws?)
Please note, for this answer I use the term copyright as a synonym for whatever law is applicable in your country that deals with protecting intellectual property rights for creative works. Some countries use quite different names for such a law.
Here's a little guide how this topic can be approached. In a professional context, I advise to consult a specialist to clear the rights for you. When you can try to create as much creative content as possible on your own. However, when you decide to tackle the issue of clearing rights yourself this is how I would do it:
Check if the copyright has been expired. Read the law (or at least Wikipedia) to find out how long this time span is in your country. Usually, it's a very long time, so you will rarely find anything from the 2nd half of 20th century or newer with expired copyright.
Find out which copyright limitations or exemptions are defined in the law in your country (e.g. "fair use" in US law). Usually you don't have to ask the creator for permission to freely use their work as parody, in an educational context or when citing an excerpt, but pay attention to quote only the bare minimum.
In most countries there should be a threshold when copyright can be applied. Sometimes the work is so trivial that it shows not enough merit to be protected by the copyright law. In some countries the threshold is very low and you have to read a lot of court decisions to get a good grasp how this is interpreted in practise. If we take a look at your example, after hearing it several times, I am somewhat sure that it is not a simple, plain recording. Either they have created the sound digitally or invested some energy in cleaning up and tweaking a recording. This probably means it has enough merit to be protected. This is my opinion based on what I know how such a case might be judged in my country and if it applies to yours too, you have to decided for yourself.
If none of the above conditions are fulfilled or you are unsure (for example in case of orphaned works), it is safe to assume that the work is protected. If you still want to use the work, check what license options are available. Sometimes Creative Commons allow you free usage of the work but you need to carefully follow every condition they make. Creative Commons comes in various versions and variants. Especially for commercial projects their conditions are often hard or impossible to fulfill. When you do not find any license information, you can always try to contact the creator and discuss business and agree upon a contract for your situation. It will probably cost you a bit but usually you are on the safe side with a contract. If you do not know much about contract law, it will be best to hire a lawyer. However, you have to make sure you are talking to the rightful copyright owner, not to someone who has just published the work with or even without permission and pretends to be the owner. For the example you have provided in the comments to your question, I have my doubt how well such an agreement works, as this person has no contact information on their channel which indicates a non-professional context. This always complicates things.
Do not use YouTube to search for sounds when you need them published along with your own work even in an edited version, like in your case where you want to built an app and use the sound there and in videos. Use trustworthy sources instead, that means professional sound libraries with clearly stated terms of service and license terms. Besides, maybe you already have access to royalty free sounds without knowing it via some software subcription you may have. For example, Adobe offers such sounds when you are a subscriber to their Creative Cloud.
I will not give legal advice how to proceed, but my general recommendation is to look for sounds at some better place, as described in step 5 above, because buying a proper license from this YouTuber person might be hard work. I guess they are not prepared for such a scenario and have no contracts ready, no license terms, etc.