We have been on the yellow bus tours in both Porto and Lisbon in the last week and I think we have the same complaint about both. The quality of the audio was poor on both. We have done hop on hop off buses in a lot of cities and nearly all of them announce up coming stops and what is available to see at the stop I.e. museums, art galleries, restaurants etc....... This didn’t happen here and proved to be annoying in the end. We will most likely try the Red Bus in the next place we go to.
There’s definitely room for improvement