#571 lastest version, opalmcu cann't play wave file

[zbyslb]

used the latest master trunk, ptlib is 2.17.4alpha, opal is 2.17.5alpha, the test program is used the opalmcu from samples, the run cmd is:./opalmcu --trace-level 5 -o my.txt -S udp\$61.232.13.245:5061 -u 51100710 -p 010888 -V -r chafei7.wg800.com --name conf
the operate is::
conf add 1111
member add 1111 sip:013910827998@chafei7.wg800.com
conf play 1111 /test/test.wav
member add 1111 sip:013661021315@chafei7.wg800.com
we can talk between 13910827998 and 13661021315, but we can not listen the wave file.

[zbyslb]

if i copy the opalmixer.h and opalmixer.cxx from opal version 3.12.9 to replace the new version, all function work ok. os i propose to use the old version opalmixer.

[zbyslb]

from 3.14 to 3.17, the opalmixer cannnt support play wave file