Concerning sight-reading notation, it actually depends on what aspect of sight-reading you are strugling with: is it the reading itself (not able to read well enough and fast enough in real time), or is it the execution (not knowing well enough your fingerboard in order to play what you read), or both? It also depends on whether what you find difficult are the notes themselves (intervals, key signatures, accidentals), or the rhythm?
Concerning chord symbols, similar question: what is your problem, exactly? Is it that you are unfamiliar with the meaning of chord symbols themselves, or is it that you don't know what notes (root, 3rd, 5th, etc.) are implied by a given chord symbol (do you know without thinking what notes are in Bbm7b5?), or is it that you don't know where those notes are on your fingerboard?
With those precisions, one might be able to offer you some suggestions about how to improve your sight-reading.
Concerning the sight-reading audition itself, although you will see the music for the first time, you are usually allowed to study it (without playing) for a few minutes, before starting to play. So, here are things you should look for when studying the piece: (if you are allowed to write on the sheet, then use a pencil to circle spots that require special attention).
- the time signature and tempo indication (look also for possible changes of time signature and or tempo)
- key signature (and possible changes of key signature)
- style indication (eg. swing, rock, latin, etc.)
- overall structure of the piece (sections, repeats, exits 1 and 2, da capo, coda, etc.)
- accidentals
- articulation and dynamics indications.
- if there are spots with complex rhythms (syncopations, rests, tied notes, etc.) sing them in you head to make sure you are ok with them.
- scalar type material will be quite easy, whereas music will lots of skips will be more challenging; play the piece mentally to identify suitable fingerings, and mark them of the sheet in all places where they are not obvious.
Good luck!