Look, everybody's tastes are different, so take what I say about fashion with a grain of salt; but it's always wise to err on the conservative side when you're going on job interviews (and let's face it, that's what an audition is).
I wouldn't wear a floor-length skirt or a maxidress to an audition. It may be adorable, but it will either come across as too casual or too dressy. Formal or semi-formal attire is ONLY appropriate for competition-type auditions where there is an audience and formal attire has been specified. Otherwise, you should wear daywear. By that I mean a nice skirt and blouse, suit, or dress. You want to look attractive, dress appropriately for your age, and not come across as too businesslike. You want to show off your figure without looking hootchy. When you're dressing, be sure to look at your silhouette --- look at your whole shape, not just at the clothes.
Most women look best in knee length skirts. If you have an issue with your legs and want to cover them up, knee boots or opaque tights with matching pumps are always acceptable (though they might be a little much in the summer).
I don't mind bare arms in an audition --- again, as long as you look professional and put-together, not like you stopped off on your way to the club or the beach. If you feel like you need a shawl, you probably shouldn't be going sleeveless. Shawls and wraps are hard to manage onstage, and they can be distracting. If you need more coverage, go for a bolero jacket or a shrug instead.
A badly-dressed singer who sings well and is otherwise on top of things is not going to lose points in my book. But if you show up looking sloppy, or have planned badly, that may cause me to call your judgment and management abilities into question. If you are completely put together, in your image as well as your presentation, then you will be impressive even if your singing isn't quite all that it should be.
And while we're at it: I actually don't mind casual attire for auditions like the ones I just did; I try to keep them fairly casual anyway because I want singers to be at ease and be able to do their best work. But you can be casual and still look put-together and fresh. For example, one young lady was wearing a summer skirt, a sleeveless blouse, and low-heeled sandals, no pantyhose. This is very casual attire for an audition, and I myself would never show up to an audition so casually dressed, but everything was neat and clean, fit her well, and suited her. I have no problem with that.
Hope that helps!
Some days it's just not worth chewing through the restraints.
