When I started Honest Hearts nobody has voices. The exception to this is during the scripted sequence right when you arrive in Zion . That's the only time anyone had voice acting.

Everyone outside the DLC still has voices, and the Happy Trails Guards had voices because they just said regular mercenary lines..

Try verifying the game cache, or deleting HonestHearts - Sounds.bsa from the Data folder and verifying again.

As PeterF, verifying the game cache may solve this. Steps:

1) Open Steam
2) Right click on Fallout: New Vegas
3) Hit Properties
4) Click the "Local Files" tab
5) Click on "Verify Integrity of Game Cache"

If that doesn't work, delete your ini files and repeat steps 1-5

this unfortunately did not work. i had previously tried what peterf said, and that didn't either. by ini files do you mean fallout_default, low, Very_high etc?

make sure you are not set for 5.1 sound etc. set sound to stereo 2 speaker. I had this problem and it was fixed when i set to 2 speakers in windows.

this did the trick!

i recently installed a crappy little sound card so bad company 2 wouldn't crash, and the utility from this was set for 5.1 mode. thank you so much!!