Are there any viable sound IC's in older consumer PC sound cards/modems for diy/circuitbending?
The modem ICs are rather limited in the sound they can generate. You have FSK (Frequency Shift Keying), which switches between two frequencies depending on the input bit's value. More advanced modulation methods include PM (Phase modulation), but that doesn't add more frequencies.
The dialler will be a DTMF (Dual Tone Multiple Frequency) generator, which generates a pair of frequencies from a limited set of eight. So that's out as well.  

Creative Labs used custom chips on its SoundBlaster boards. They may be very versatile, but unless you can find a datasheet they're useless.  

I looked around a bit and found the SSG01 Sound Coprocessor IC, which may also be interesting. (Yes, I know you're broke.) It has a simple interface: it's controlled via EIA232 and outputs a mono audio signal. Unfortunately the link to the datasheet on the page is dead.