Traffic is load-balancing over both links, causing packets to arrive out of order. This can cause voice quality issues, such as jitter and delay. To avoid this problem, voice traffic should be sent over a single path, using a routing protocol that supports unequal-cost load balancing, such as EIGRP. The source of this answer is the Cisco ENCOR v1.1 course, module 4, lesson 4.3: Implementing EIGRP.