Why sound theme only available in firefox and thundeird

[yangdawei]

Environment:  Xfce4.14 and Debian10

Problem:

I followed all guidelines such as 
https://forum.xfce.org/viewtopic.php?id=8618. However the sound theme is 
only effective in firefox and thunderbird. It sounds no any voice in 
system and other applications.

How can I enable the sound in all system? Please show me with your kind.

-- 
Yang Da-wei
Email: yangdawei.hit at qq.com