First of all, you need to fix the settings. Make sure the speaker is on. Even if you are hearing something, it does not mean that the speaker is turned on. The volume you hear might be coming from the earpiece, and will be very low indeed.
Every phone has a dedicated key for the speaker. When on call, the phone displays a special option to turn on the speaker. You can go ahead only after turning on the speaker properly.
The first step to raise the volume is to check the external controls. Most of the phones do have external volume controls usually at the left or the right side.
Use these buttons to increase the volume. Remember, these keys will also raise the handset volume. However, a few phones don’t change handset volume if you are using the keys after turning on the speaker.
Your problem will be solved by the external controls. However, if the problem still persists, you must open the ‘Sound’ settings. After entering the sound settings, look for the option that says ‘Volume’. Dial it up to maximum.
The internal controls continue to work, even if you have external controls. There must be separate options for ring, alarm and headset volume. You can adjust the volume of each function separately.
It is always better to keep the volume level to maximum, when using the internal settings. If you feel that the volume is too high, you can adjust it with the help of external controls. In case both internal and external controls don’t help your cause, there must be some hardware issue with the speaker.