You could take a look at the
speaker entity. It plays a sentence from
sentences.txt at a random interval between 3 and 10 minutes. Using a sentence group (sentences with the same name but different number suffix) will make it choose a random sentence from the group.
It's used a lot in the original Half-Life campaign where it adds random background noises. In these cases multiple
speaker
entities are used together to make the sounds play more often. An example would be c2a5c, where four
speaker
entities set to play the
FAR_WAR sentence group to add an ambiance of distant combat.
If you're mapping for HL, you can take a look at predefined sentence groups in sentences.txt and see if any fits your need. If you're making a mod, you could create a new custom sentence group that fits exactly your need.