Actually now that I think about it, this issue may also apply to IM and may also exist prior to changing the sounds; I've attempted to use the IM before and other users do not seem to get the messages (even though I have site-wide checking enabled).

Update: I did an actual test with another user. This bug does not apply to IMs, nor does it apply to guest users on chat rooms. However logged in users in the chat room will get the sound notification of a new message, but will not see the new message unless they refresh the page or send a message of their own.

Reproduced on your site, can't on stock v9. Investigating.

It's a conflict with site-wide IMs. It's more than just welcome posts, it's all posts not coming through. Site-wide IMs are only for members, hence no issue for guests.

Please try uploading the attached template to themes/default/templates/.

Should I upload it to my custom theme as well?

Nope just fix default template. Only would need to change in theme if overridden already in theme

Alright thanks Chris. It's already overwritten so won't change it.

EDIT: I mean the change already placed in custom theme

Alright this is to confirm issue was resolved. Thanks again ^^