Simple answer, if you use copyrighted material, you must pay a license.
As regard to songs, that generally means licensing them directly from the owners/publishers. However that simply gives you the license to include the music in the game or to distribute, in all likelyhood you would also need to pay royalties for 'performance' (e.g. playback) of the song. This is rather complex as a number of companies exist to deal with licensing/royalties.
As for models and other assets, again you'll need to buy a license to use them in your game, unless they are released for free/creative commons.
In either case, just because you are releasing a webgame, or aren't charging for it means nothing, you still have to get the licenses, else you will at best be shut down at worst sued.
There is a possible workaround for the music, that is to simply not include any songs. Instead you would provide the data file for a file (i.e the note chart) and let end users access their song library. Of course there are issues such as many different versions of a single song in a verity of formats, from official releases to cd rips or pirated, which might require multiple note chart data files to support.
