This is a small app aimed primarily at the Xperia V with <= 4.1.2 on it but I'll include the other Sony devices in case the behavior is similar on other products.
The app is a "Small App" which appears in the Small App tray accessed by pressing the Recents button. Then, while in a phone call in speakerphone mode, press the Recents button and load the app. Done!
The app engages the internal loudspeaker during speakerphone mode (aka Handsfree, Handheld hands-free, Hand-held hands free, HHHF, etc). The V does not always route the Rx audio to the internal loudspeaker and instead leaves it routed to the receiver/ ear piece/ etc. The HFWSA attempts to reroute the incoming audio signal to the internal speaker.
Here is some text that http://developer.sonymobile.com/knowledge-base/sony-add-on-sdk/small-app-api/ would have me include: "[ SmApEx4SoPr ] Small Apps extension for Sony products".
I got the icon from iconfinder.com and it was made by Matthew Inman - http://matthewinman.com/ .