This is all going to depend on how capable you are as an audio machine and what your budget is.
If you do it yourself, you can either find some tools that let you generate sound effects or you can record your own from coconuts or something. If you try recording your own I would highly recommend you try renting or borrowing some nice audio equipment, since the better quality the audio to begin with the easier it will be to post process. Combine two home made sound effects to get one unique sound effect, for example! For music then, see if you can get some easy music making software that allows you commercial rights to the tracks. Overall, making audio yourself can be a painful process for you if you aren't experienced with audio at all and you might not produce good results after hours and hours of effort. On the plus side, this should be cheap if you don't value your time.
If you hire someone to do the work for you. It's possible you might have to hire several people to do the work since someone who is good at music might not be good at sound effects. Either way, this is going to cost you money but going to be the best for you in the long run if you are not very capable at producing the audio yourself. You will save yourself the headaches from not having to make it yourself. This route will also give you the freedom to be productive on areas you are good at while the contractor focuses on the audio. A professional should have the appropriate skill set and equipment (or means to acquire the equipment) to handle producing of the audio for you. In the end you should end up with a high quality product in exchange for a nice sum of money.
As a last alternative, there is royalty free audio out there in the world. You can explore the public domain or do some investigation via Google and maybe you'll be able to find something that suits your game, or at least find multiple sound effects you can modify to make work for you which will take significantly less effort than creating your own from scratch.