Core issue
This issue is documented in Cisco bug IDs CSCsf03420 and CSCsd69887.
Voice traffic fails to pass bi-directionally when VPN Client software versions 4.6 through 4.8 connect to the VPN 3000 Concentrator/Adaptive Security Appliance (ASA)/PIX using Advanced Encryption Standard (AES) encryption.
Resolution
As a workaround, choose from either one of these two options: