The Yamaha demo site has some accordion sounds in the World demo in the Sound Showcase which you will find in the Demonstration section.
http://aux.music.yamaha.com/tyros2/ Others may chime in with more relavant demos.
I have found the T2 accordion sounds to be usefull. The master accordion is a little bit heavy for me but the Jazz is useable as is the Mussette. I am not knowledgeable enough about accordions to tell you which has a servicable cassotto sound
As for violins, the solo violin is nice as are several of the general string section/orchestra string voices. What I find missing is a small ensemble sound, like a quartet.