What Roland said except the prices may be much lower.
Amazon lists RCA composite/RCA stereo units for under $10. Beware of the units that are USB-powered, as you'll likely need to provide the source.
Optical may necessarily involve a loss of surround sound (employing an audio extractor to get to RCA). The DMCA puts a serious hurt on digital-to-digital conversions, whether or not the original content was somehow copy-protected.
Providing more specificity about your use case (e.g., gaming console, LaserDisc player) may yield a more targeted answer.