My old copy of Curse of the Azure bonds had music on the title screen - or at least if you let it idle a while. Granted it was somewhat painful pc speaker music, but the gog.com version doesn't seem to play it, although sound effects work. I'm playing the Linux version.

Unfortunately the introduction music is broken in Curse of the Azure Bonds 1.3, which is the version used by GOG. All previous versions of the game offered the introduction music for PC Speaker and Tandy.

I posted message about broken music in the release announcement thread, but there has been no comments about the issue or any fix been offered.

From ChangeLog today: Curse of the Azure Bonds version 1.2 added to the bonus goodies section on 14.09.2015.

This version has intro music.

Now we just need Pool of Radiance with intro music!

Yes 1.3 versions of Pool of Radiance and Curse of the Azure Bonds shares same problem .

Out of curiosity, are there any other differences between 1.2 and 1.3 for either game?

(In particular, I am wondering if there are any interesting bugs that got patched out in 1.3.)

Only Pool of Radiance 1.0 (and maybe 1.1) offered the introduction music, which are going to be more difficult to find. It looks like only the original disks releases offered the early versions, and not any of the re-releases. Even the first game compilation (Advanced Dungeons & Dragons Limited Edition Collector's Set) used Pool of Radiance version 1.2. There are no differences between the data files in Curse of the Azure Bonds versions 1.2 and 1.3, only the executable file was updated. So only minor bugs fixes in code at the most, but I wouldn't be surprised if they only disabled/removed music code in version 1.3.

Why they need to disable music ?

When were the latter versions released? Maybe music through the PC speaker was considered antiquated by then, and silence preferable.