Click this link: https://www.duolingo.com/settings

Yes, I believe you can go to the settings and change it. I disabled mine already.

Yes, you can disable speaking exercises by turning off the microphone in your "Settings" https://www.duolingo.com/settings

In pictures:

• https://support.duolingo.com/hc/en-us/article_attachments/202621934/settingsmenu.png

• https://support.duolingo.com/hc/en-us/article_attachments/202621944/settings2.png