As l work in my house or walk in my neighborhood l can hear minyanim that are saying kaddish, borchu or chazoras hashatz. Do l have to stop to answer?

There is an obligation to answer Amen whenever one hears any Beracha. You do not need to stop to listen to chazoras hashatz when walking outside, but if you hear a Beracha you should answer. Also no need to answer Borchu, Kaddish, etc. (it’s a good idea to answer at least once for the mitzva).

The Halacha is different when one is in a shul.