Can old Airport Express be used for Music Streaming on the stereo amp?

Hmm. I'm only aware of three AirPort Express base station models:

• A1084/A1088 - 802.11g that came out in 2004.
• A1284 - 1st gen 802.11n that came out in 2008
• A1392 - 2nd gen 802.11n that came out in 2012

Regardless, all of them came equipped with a 3.5mm analog/digital optical audio port. Not sure why yours does not. Are you sure that it is an AirPort Express, and not another AirPort model (Extreme or Time Capsule?)

The Express is a wireless router. You would send an AirPlay stream to it either by a wireless or wired (Ethernet) connection, and in turn, the Express would send that stream out of it audio port. It has no ability to send an AirPlay stream out ... only receive them.

Yes, that would be correct. Only the Express base station have, not only an audio port, but the required audio circuitry to process the AirPlay stream.

The Extreme does not have either of these. However, since it can provide a wireless network, an AirPlay stream can travel over it.

By "stereo amp" are you referring to a combo device, like a receiver or a pre-amp/amp? For either it would have to have either a analog or digital optical input port to access the stream from the AirPort Express base station. If it does, then it should only be a matter of connecting the appropriate cable type between them.

if it does not have 3.5mm minijack how would it play audio ?