Codes are sometimes written by previous owners on the head unit casing, so it's worth pulling the unit out to check. Otherwise it's either a dealer (assuming the radio is original to the car - you'll need the VIN) or one of the many decoding services advertised in various places.
Upgrades? Well it depends on what you have and what you want. If you have a CD, tape and radio then you have just about everything that was available, except the CD multi-changer (boot mounted unit) or the 3-disc version of the dash-mounted offering.
It is possible to hard-wire an aux lead if you're handy with a soldering iron, or you can buy plug-in aux adaptors on Ebay.
In our 97 S90 and our 98 S70 I've simply added the Kitsound FM transmitter units that connect to a Bluetooth phone. These are OK, but not 100% brilliant. We got ours from Halfords they plug into the lighter socket and are very easy to use. They also have a hands-free phone function.
I've looked at more modern audio units, but it seems that anything in the £100 - £300 bracket is utter rubbish, and is hopelessly out-performed by the original Volvo-branded Alpine units. The OEM kit is probably amongst the best car audio ever.
Hope that helps
Jack
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