In the sound effects world, SFX libraries are buy-outs… you pay for the SFX library and you are given the rights to use for SYNC to picture or as a part of a mix for audio. You are not allowed to repackage or re-sell the Sound Effect in other ways. Some sound effect libraries also have special requirements for game audio as well, as some games utilize a database of sounds within the game and the sound effect is unaltered and delivered to the game company as a non-sync sound. Sound Effects libraries or individual sounds purchased on the net are different than music which can be royalty free buy-out, or needle drop, and other license methods.
Each music library license will vary to a certain degree but as a general rule royalty free music simply means that you have purchased a “lifetime sync license” for a given song or group of songs. In other words, you have the right to synchronize the music with your audio and/or video productions an unlimited number of times without incurring any additional expense.
This “life-time sync license” is normally how sound effect libraries work. buy the library and use the library forever as long as its used within the rules of the license.