[LAD] Is there a Linux Software Mixer?

Hi All I think I know the answer to this, but I was wondering if it's possible to mix two separate audio sources in software. I'd like to be able to play announcement-type audio atop the currently playing main track. An example might be when driving down the Autobahn and a traffic report temporarily pre-empts whatever it is you are listening to. In my case, though, I'd prefer to keep the main audio playing, but at a very low volume level compared to the short announcement.. My setup is a roll-your-own embedded Linux distro with kernel 2.6.28. I'm using Alsa SoC audio driving an I2S output on my Marvell processor (which is now working just fine, BTW). The I2S signals feed an FM transmitter chip which is broadcasting audio to a nearby FM receiver. The FM chip only has the one input so no mixing possible within that part. Anyhow, considering I only have a single I2S output, seems to me that I'd need to do the mixing somewhere upstream of that serial port within the Linux machine. But there is no special hardware for this and I think the answer is "no, there is no way to mix two separate audio tracks without a DSP and another device driver". But I thought I'd ask just the same. Regards, Rory