On all four maps in custom games there have been no Voice or Announcer tracks. I tried tweaking them in the sound control just in case they got rebalanced wrongly, but that didn't change anything.
The individual sound effects are still there, just not the voiced parts. With Runeborn Xerath, the sfx played for his joke, taunt, and death, but nothing from the voice line. (Xerath had no sounds for his kit, but I figure that the assets are coming soon.)
Kog'Maw didn't have any sound when firing his Q, but there was a sound for when it hit. I figure this also is a case of assets still in the pipeline rather than a bug.
With Heimer, all the kit sfx (but no voice) were there, which is why I think the Xerath/Kog kit sfx are works in progress.