This generally works if the speaker sound is very low but you can hear it. If you can't hear anything then you might need a speaker replacement but it wouldn't hurt to check to see if its cleaned.
The way I clean phone speakers is with a toothbrush, isopropyl 99% alcohol and compressed air.
# Look at speaker to see if you can see anything blocking it
# Take dry toothbrush and brush the speakers using a little pressure but not much (you may see stuff come out keep going for like a minute)
# Puff the speaker area lightly with some compressed air
# Test call to see if sound improved
# If not, Put a couple drops of isopropyl alcohol on to toothbrush and complete step 2 and 3 again.
# Test call to see if sound improved
# If not, put could of drops of isopropyl alcohol into speaker area and plus a medium stream of air into speaker area not to powerful, then follow up with one minute of brushing.
# Test call to see id sound has improved
# If following this doesn'twork then more than likely you need a speaker replacement.