Unlike the HTC One and Xperia Z2, the Samsung Galaxy S5 only comes with a single mono speaker at its rear. While the speaker does a pretty okay job, it can still be hard to hear it sometimes, especially while on call using the speakerphone.
An XDA member, stalli0nUK, was also facing the same issue and decided to take matters into his own hand. The result? A mod that boosts the speaker output of the Galaxy S5.
The mod is meant only for the G900F variant of the Galaxy S5 and requires root access with a custom recovery installed. It contains some modified system files that help in boosting the overall speaker output of the Galaxy S5. Do keep in mind that installing the mod might reduce the longevity of the speaker on your Galaxy S5, even though users have not reported any damage until now. Also, you might notice slight distortion while playing back music in certain songs because of the speaker being pushed to its limits.
Either ways, if you are frustrated with the low speaker volume output of your Galaxy S5, head over to this XDA thread to download the mod and to get rid of your woes. A similar mod also exists for the new HTC One (M8) and the Nexus 5.