I also have ATI video card with HDMI output and tried everything i could to output sound to both PC speakers and TV with HDMI simultaneously, i spent nights doing research online i found some guys method of using "stereo mix" in recordings settings, but my sound card didn't have the option to "listen" to another device, so what worked for me was this method: (please bare in mind it did cost around $6.00) but well worth it!
Items required:
1) 3.5mm splitter (you can find one on ebay for about $2.00)
2) 12ft 3.5mm m-m (depending on how far your computer is from your tv - the cable was $4.00)
Once you obtain these items, connect the 3.5mm splitter to the back of your PC (green jack), then connect your pc speakers to one end of the splitter and then male-to-male extension cable to the other end of the splitter, connect the other end this m-m cable to your TV.
Go to your control panel, system, device manager and inside "Sound, Video Game Controllers" Disable your ATI HDMI audio output, this will trick your tv into thinking that you don't have sound on your HDMI cable and will let you use the 3.5mm phone jack to output audio instead.