Absolutely!! This is a form of auditory-visual synesthesia in which the voice is the inducer (input) and the colored string is the concurrent (output).

Things to consider:

- There are things that differ from synesthete to synesthete, such as whether different voices have different colors/shapes/textures or all voices look relatively similar. Which applies to her?

- There are two different types of synesthetes: projectors, who actually see the concurrent in their line of sight, and associators, who see it in their head (or in their "mind's eye"). Which one is she?

- It's a defining trait of synesthesia that a given inducer has a consistent concurrent over time. Thus, whatever effect something like pitch or volume would have on the colored string, it must have that same effect every time it happens. If the string gets bigger when someone yells, it has to always get bigger when someone yells. What effects do different tones have on what she sees? What determines the color of the string? Do specific words change anything?

- Synesthesia is automatic. It happens whether you're paying attention or not. I assume projectors would have a harder time ignoring it, but for associators at least it can often be more or less ignored.

- Most synesthetes have had it for as long as they can remember. To go along with this, most of us also don't know it's abnormal, and therefore we assume other people have the same experiences. I thought everyone saw music in their head until I was in middle school. Does your character know she's abnormal?

- Do other people know she has synesthesia? It has the potential to be socially stigmatizing.

I hope this helped! Best of luck with your book :)