I've had a similar problem from time to time and it is usually due to the rubber ear piece not seated properly on the speaker. Sometimes the "lip" that attaches to the speaker gets pushed down and the rubber earpiece ends up pushed against the speaker. If you're fairly certain that it is not your earpiece, you can take it off and hold the Era in position to verify if it really is the speaker. If it is still muffled, there might be something physically damaged and it would be worth contact support.
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