Why can I watch youtube videos on my old laptop but not on my new one?

I'm not sure. Try clearing your web browser's history and cache. At the school district I work for, we have found that regularly doing this will solve a fair amount of website related issues.

There are tons of guides that show you how to do this on most web browsers out there. One Chrome,Firefox, MS Edge, you can usually press Ctrl+Shift+Del simultaneously to bring up the clear browsing data menu.

it's broken. take it back for a replacement.

Difficult to say - could be a lot of things. 

Mac OS or Windows, or Linux?

Other websites loading?

Other sound working?

I would start with simply clearing the browser cache within Chrome, Firefox, Edge, or Safari - or whichever browser you use.

You can Google how to do this in your particular browser.

Over time as you navigate to various websites, your web browser stores stuff for later retrieval and to speed up your browsing. Sometimes this 'cache' can get messed up and manually deleting it, can help.

It has been "Tech Support Step 1" for over like 2 decades. - one of the 1st things any Tech Support person will walk you thru.

We cannot provide an actual answer unless you tell us the brand and model of your laptop. Laptop manufacturers preinstall their own applications. Whether or not you think they are beneficial is up to you.

However, you should perform diagnostics by running troubleshooters. Updating the drivers may help resolve your issue.