You mean the game doesn't play any music at all? I have a good guess as to what's happening!
When Yserbius was programmed, it was envisioned as an online-only addition to The Sierra Network. It was only a year later that the single player port was created. So it would have been installed on top of an existing installation of TSN, most likely at C:\TSN\YSERBIUS. To a certain extent, that would allow it to piggy back off of any setup that the game's owner would have already done for TSN. For example, there would be no need for the (again, online-only) Yserbius to ask what kind of sound card you had in your computer and store your response in its own folder; instead, Yserbius would simply read the sound card setup files in the parent (".\..\") directory.
When the single-player version was released, they didn't change this behavior - the single player client still looks for the sound card setup files in the parent directory.
I'm guessing that your dosbox installation includes only Yserbius, not Yserbius as a child folder of the full TSN/INN install.
TL;DR:
If you install Yserbius as part of the whole TSN/INN shebang, I'll bet the game will start singing to you.