Relevant effects:
Challenge: +2 to attack.
Bardic Performance: +2 competence bonus to attack & damage Rolls
Haste: +1 to attack
Dual wield: -4 to attack
Rolls
Main Attack (frost hammer) - (1d20+11, 1d8+8, 1d6)
1d20+11 : (12) + 11 = 23
1d8+8 : (1) + 8 = 9
1d6 : (5) = 5
Off-hand Attack (cold Iron hammer) - (1d20+11, 1d8+6)
1d20+11 : (7) + 11 = 18
1d8+6 : (6) + 6 = 12
Haste Attack (frost hammer) - (1d20+11, 1d8+8, 1d6)
1d20+11 : (4) + 11 = 15
1d8+8 : (1) + 8 = 9
1d6 : (5) = 5