1. Speaking at a high volume: This is the most common meaning. It simply means to speak with a loud voice.
Example: "Please don't speak so loud, you're giving me a headache."
2. Speaking clearly and confidently: In this case, "loud" refers to the way the speaker projects their voice, not necessarily the volume. It implies they are confident and assertive in their speech.
Example: "He spoke loud and clear about the importance of the project."
3. Speaking forcefully or aggressively: This meaning emphasizes the speaker's intent and tone of voice. They are trying to get their point across with emphasis.
Example: "She spoke loud and angrily about the injustice."
It's important to consider the context to understand the intended meaning of "to speak loud."
It's also worth noting that the phrase "speak loudly" is more common than "speak loud."