Generally, if you live and/or work overseas, the IRS will either send you an Audit Notification Letter or make contact by telephone.
A written IRS audit Notification Letter may be very confusing to you as to what is the nature and scope of the Letter you have just received. Notification Letters are designed to be short and provide the minimum of information for privacy purposes.
However, there will be a telephone number listed on the Letter and you may want to first contact that person to obtain information to ensure to this is not part of the many hoax letters, emails and telephone contacts that are made to US taxpayers.
Once you have satisfied yourself as to the validity of the Notice, then you should immediately contact your Tax Counsel, such as a tax attorney, to act on your behalf.
Remember, anything you say or communicate to the IRS will be used by them in a manner that is not advantageous to your position(s) for sustaining that a valid, correct and true tax return was filed with the IRS.
Note, the time given for you to respond is now shortened to days, generally 10 calendar days, to provide a massive list of documentation and information. An experienced tax attorney can get this deadline extended when a US taxpayer can not. You will need the time to find out more about the IRS inquiry and to supply the proper and organized tax documents they are seeking along with a well reasoned explanation and position.