Yes, in fanfiction, the possibilities are endless. Ash could be depicted as having a great singing voice. Writers often take creative liberties in fanfiction to show characters in new and unexpected ways, so it's very likely that there are fanfictions out there where Ash sings.
Since I'm not familiar with this specific 'sing ash crying fanfiction', it's hard to say exactly. But generally in fanfiction, common themes could be love, friendship, or struggle. Maybe in this one, it could be about a character named Sing or Ash going through some emotional turmoil and crying as a result.
You can try fanfiction websites like Archive of Our Own (AO3). There are a lot of different fanfictions there, and you might be able to find some great 'Sing ash fanfiction' by using the search function.
You can try websites like FanFiction.net. It has a large collection of various fanfictions, and there's a good chance you'll find some for 'Sing Ash and Lance' there.
Perhaps a fanfiction that focuses on their friendship growing over time. They start as acquaintances and through various events, like helping out at a local fair or rescuing a lost pet, they become the best of friends.
There's a 'Sing ash fanfiction' that focuses on Ash's backstory. It delves deep into her past, like her struggles before coming to the theater in 'Sing'. It gives a whole new perspective on her character and her relationship with the other performers. It's really interesting as it weaves in details that the movie only hinted at.
Another theme could be about their growth as musicians. How they inspire each other to improve their music, try new styles, or overcome stage fright. For instance, Johnny's discipline might influence Ash to be more focused on her music, and Ash's creativity could inspire Johnny to break out of his comfort zone.
Sing Street is not based on a true story. It's a work of imagination that brings together various elements to create an engaging and entertaining narrative.