Connecting an old Mac to home entertainment system
Yes.  It's very easy.  You need to get a "DVI to HDMI with audio" adapter.  The way it works is that it takes the two separate signals, digital video and audio, and combines them on the HDMI signal which supports the audio.

Here's one from Startech that worked really well for me.




If you notice, the DVI and RCA Audio is on the Input side and the HDMI is on the Output side.  These are not bidirectional; meaning it will not work in reverse.  So, it will take a DVI and audio and convert to HDMI.  It won't "break out" HDMI to DVI and audio.