mount small pcb parts

Hello, I have a subwoofer that stopped working, one of these two parts has failed, the big one I have found as there are markings on it, the little one I have no idea, can anyone help me identify it?

All, I am having difficulty mounting some of the small pcb pieces such as buck/boost converters (see attached figure), arduino nano, etc. My question is can I use thermal paste/grease to use as glue to mount them on the casing since they are supposed to be electrically non-conductive. Any...