I would recommend hiring a CPA or an enrolled agent to represent you in the audit. The tax professional can act as a gatekeeper so that the IRS only receives the information they have rightfully requested and they do not try an 'phish' additional clues for possible additional audit issues.
If you deal with the IRS yourself, you need to make sure you are responding to the IRS when they request documents or explanations with only the items they are requesting. If there is confusion as to what the IRS is requesting, call them, write to them for clarity.
If you don't do anything, the IRS will disallow any deduction that you do not document and they may assess a great deal or tax, penalties, and interest.