Supposedly Flashy Flash and Speed of Sound Sonic were trained in the same ninja village. Are there other ninjas from there in the series?
In the webcomic, there is currently (as of the time this answer was first written) nothing that unambiguously associates Flashy Flash with Sonic in any fashion. There are some things to suggest it, though:
When Flashy Flash fights Garou he uses a technique called "Shadow Steps" to move even faster, and that kind of sounds like the name of a ninja-based move. He also uses "Wind Blade Kick", a technique we saw Sonic use against Saitama in Chapter 15.
On the other hand, in the manga there is Gale Wind and Hellfire Flame, members of the Monster Association. These two are explicitly said to be from the same village as Sonic, and more recently it has been made explicit that Flashy Flash hails from this village as well.
These characters do not exist in the webcomic in this form. They are evidently based on the monsters Gale and Hellfire, who are also brothers that team up to fight Flashy Flash. But these have radically different character designs (they're robots, for one), and are quickly dispatched by Flashy Flash without receiving any real backstory. These monsters were originally designed by another mangaka, so they couldn't be used in the manga adapation due to copyright issues.
The situation in the webcomic has since changed, and there are at least 23 living ninjas we have now seen, not including a mysterious "him". Flashy Flash and Sonic are now explicitly revealed to be part of the 44th Ninja Class, same as in the Manga, as well.