This sounds like your outer speaker may be broken; this may have been from trying to play music that the speaker physically couldn't handle, or even dropping it wrong. Either way, it's a fairly easy [guide|31190|replacement] and your speaker should be good as new afterwards.
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If that doesn't work, try our [https://www.ifixit.com/Wiki/LG_Optimus_G_Pro_Troubleshooting|troubleshooting] page for more solutions.
This sounds like your outer speaker may be broken; this may have been from trying to play music that the speaker physically couldn't handle, or even dropping it wrong. Either way, it's a fairly easy [guide|31190|replacement] and your speaker should be good as new afterwards.