If you have searched on this then you will have seen more than one thread on it. Solutions vary but they all put pressure on the speaker. The speaker has two pressure type contacts and they appear to loose contact pressure after time raising the impedance of the speaker and lowering the output. In my case, I removed the battery cover, battery and the back of the Bold. The back is secured with six screws. When the back is removed the speaker contacts can then be cleaned with a contact cleaner and bent slighly up to increase the speaker contact pressure. Do not use any abrasive on the contact as this will remove the plating material and increase the likelihood of corrosion.
The other solutions involve putting material (paper?) between the speaker and the back- either under the battery or on top of it. When the battery cover is put in place this puts pressure on the speaker and pushes it on the contacts.
Please note the first process (removing the back) will negate your warranty. If you have a warranty use it!