Ambient sounds in Heretic

How do you get the ambient sounds in Heretic to play when making your own Heretic level?

There should be selections for the sounds in whatever editor. Just add them like you would any other thing and it should work. In the original .exe, you can't put too many sounds in a map or it will crash back to DOS. I don't think that's a problem in most source ports.

Right. In Heretic, ambient sounds are controlled by mapthings 1201 through 1211 (IIRC). You can place up to 16 of them on a single map (IIRC). Placing more than one of the same sound object causes that sound to have a greater probability of playing at each ambience event. In other words if you put 3 beast growls and 1 bell, the beast growl gets an effective 75% probability of playing at each event, whereas the bells only have 25%. Raven programmed it this way intentionally, as they use it in their own maps this way (E1M1 is an excellent example, with multiple drip objects).

In Eternity, there is no limit on the number of spots for one map, the ambience sequences are defined via EDF and can be overridden, and you can use objects 1200 through 1299 to refer to the first 100 sequences, and object 1300 can be used in either the Hexen map format or via ExtraData to specify any sequence number in the mapthing's Arg1 field (in Hexen this would be limited to 0-255, but via ExtraData the limit is 0 to approximately 2 billion).