I filed early and have since received a 1099 in the mail, which I did not include when I filed. Am I risking an audit if I don't file an amended return?
My advice is to file an amended return if the 1099 shows income that you have not reported. If you do not amend your return, whether or not you are audited, you are likely to get a recalculated tax return from the IRS, with an interest charge and, possibly, penalties on any unpaid tax. If the additional income is immaterial, then you may consider asking the IRS to recalculate your tax and writing a check for any additional amount owed, which could be cheaper than the cost of amending.
I would definitely amend it. You can use a fillable form 1040X from the IRS website and mail it in. I have done it several times for several people and its very simple to execute. The IRS even provides you with an explanation section on the form.
Avoid the risk and of an audit or tax penalty. File amended returns with both the IRS and your home state. If you prepared your return on your own using a consumer software package, there should be a way to file the amendment there. A tax preparer will do this for you as well but expect to pay a fee (less if they already handled the initial filing). Some states provide online resources similar to the one referenced by Mr. Patel earlier that may help you do it yourself if you choose.