Miles are always returned to the account from which they paid. Taxes and fees are refunded to the original form of payment however unless there was an covered reason (i.e. significant schedule change, change of operating carrier, change of aircraft not acceptable to you) and or you are not an AA Executive Platinum, a mileage redeposit fee will be assessed.
Depending on the card brand/type used to pay for the ticket issuance, a covered medical reason may give you a credit from your card issuer to offset the redeposit fee.