WACC (IRR FOR IRRDEMEEMABLE/REDEEMABLE)

Hi john.i am confused about which discount factor rates should be selected for NPV (IRR calculation).is the general idea behind use lower rates from coupan rate?
For example if loan notes are 12% redeemable.cash flow should discounted on 5% and 10% for IRR (rates are lower than 12%) for WACC or individual cost of capital calculation.please correct me if I am wrong.