So the other day I installed a Radeon HD 4870 and a new power supply and well it plays the audio through the dvi/HDMI converter but my problem unlike most is now it won't play through stock sound jack that came with the computer rendering my computer speakers useless. To be honest I wouldn't mind just using the HDMI on my tv but my girlfriend and I usually like to switch playing games on the computer to tv separately and we both can't do that with the hdmi. I just want the sound back on my computer speakers.
I had a similar problem. I istalled the xfx HD 4870 and a new PSU in my HP Pavilion and the realtek sound on my mobo stopped working. Here's the solution (at least it worked for me). When your pc boots, go into BIOS. There's an advanced setting for audio. Change the setting from auto to enabled and save. Apparently if is set to auto, the system will choose for you, and in this case it will always choose the sound hardware in the HD 4870. After windows boots, you may also have to go to control panel/ sound and select the realtek audio device as your default. This fixed my problem.