The reason why this isn't possible like you wish is because the custom sounds always need to show the text which is currently shown (in order to let the contact and you perform the specific functions of downloading, playing, storing the sound)...
The only thing you can do is to use QuickText to combine the command for a custom sound and a specific text (Note, you can even use the (!P) tag to provide a parameter as text...
QuickText Alias:
/mytextsound
QuickText Commands:
Hello there
#/sound #123456789AB
Now you can use the mytextsound command to both send the sound and text automatically. Or even with a parameter:
QuickText Alias:
/mytextsound
QuickText Commands:
(!P)
#/sound #123456789AB
Now you can use the mytextsound command followed by your text you want to send, like
/mytextsound blahblahblah.
All this can be found in the
official help pages.
Also see
CookieRevised's reply to Link Custom sound to Word???.
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Other than that, the only thing you could try is to use one of the
very old sound plugins (but I actually doubt you can send text with them also). But both you and your contact will always need to have those plugins and sounds installed.
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