Please try selecting a different driver form "Configuration" -> "Audio" -> "Audio Driver" and test to see if there is improvement in the quality.
Also if you experience lack of audio or one-way audio due to NAT, altering the STUN and rport should help.
STUN is located in "Configuration" -> "Accounts" -> your accounts -> "Network Settings".
If using Zoiper with Wi-Fi, please make sure that the routing device is not blocking ports used by Zoiper. The default ports used by Zoiper are:
-SIP port is 5060
-IAX port is 4569 UDP
-RTP port is 8000 and above UDP
Default STUN values are:
Server hostname/IP: stun.zoiper.com
Port: 3478 UDP/TCP
Refresh period: 30