By wave drivers I mean the voice/wave drivers that come with every voice modem. They allow the modem to act as record/playback devices in windows.
We have some other vaugue reports of playback quility decreasing over long playbacks and we are researching this issue.
It may be better to try a wave file instead. If it is only speech try the GSM codec which is very efficient at voice only information. Otherwise if you have plenty of disk space try PCM.