If you are getting talked over in meetings, the first question is not how to speak louder. It is whether the meeting is even the right format for your ideas. Maybe you are better at diagramming. Maybe you are better in writing. Maybe you are at your best explaining something one on one. Meetings are just one place where decisions get made, but decisions get made in all kinds of formats, and the people with the most influence usually pick the format where they are strongest instead of forcing every idea through a conference room.
If it does matter that your voice be heard in the meeting itself, then you have to make the most of the most valuable thing everyone in that room owns: their attention. Attention is not given out as a courtesy. If you want it, your ideas have to be worthy of it. That means formatting what you say so people recognize you are addressing problems they actually have, and doing it succinctly enough that nobody gets bored waiting for your point to arrive.
You also have to give credence to other people. People listen to you when they know you have listened to them. If you have a record of hearing others accurately, acknowledging their points, and building on them, the room extends you the same treatment. If you have been transmitting without receiving, being talked over is the room returning the favor.
So if you keep getting talked over, run an honest analysis with two branches. Is it that certain people are simply overbearing, in which case you need techniques for holding attention against them? Or are you doing something that signals to other people that you are not worth listening to? The two problems have different fixes, and most people never ask which one they actually have.
The IBC course treats attention as the scarcest resource in any room and teaches you to earn it: pick the format where you are strongest, address problems your audience actually has, keep it succinct, and give others the listening you want back.