Contrarily to organizations such as the WTO, the power is skewed towards developed and major countries like China, which also have the most voting power. There seems to be a lack of effective participation by many of the developing countries, Sub-Saharan African countries in particular, in the decision making processes of the Executive Board. Because I don't know how the Executive Board operate, I don't know how developing countries can put their concerns onto the table or get their views taken into account during board discussions at the IMF.
You seem to believe the IMF is (or should be) a democratic institution. It isn't. It is a shareholder-controlled monetary institution managed by those who paid into it. That is why rich countries get one member each while all of Africa gets to select two. Groupings of countries wich represent slightly over 3% and less than 2% of votes each select one Executive Director while e.g. Saudi Arabia alone has more than 2% of the votes.
So to your question, they get heard through their regional representative.