Is the reason that pts with tracheostomies are at risk for aspiration because they are not used to breathing in and out of their noses and therefore the epiglottis does not cover the opening of the larynx?
Patients with tracheostomies don't breathe through their nose, they breathe through the tracheostomy tube. Generally speaking, the anatomy of the upper airway should be intact for most patients with a tracheostomy. As such, patients who are neurologically intact should be able to protect their own airway.
I'm also confused about cuffs and why they are there.
The cuff serves two purposes. First, if the patient is being mechanically ventilated, the cuff is frequently inflated to prevent an air leak around the tracheostomy tube. Also, the cuff might be inflated to help prevent aspiration, although it is still possible to aspirate around the inflated cuff.
Also don't understand why you uncuff the trach when you put a speaking valve on.
While the cuff is inflated, exhalation occurs through the tracheostomy tube. Deflating the cuff when the speaking valve is placed on the tracheostomy tube, occludes the tracheostomy tube. This allows the exhaled gases to exit through the larynx. It is important to remember when placing the speaking valve on the tracheostomy tube, that the cuff must be deflated, otherwise the patient will not be able to breathe effectively, if at all.
You might find the National Tracheostomy Safety Project
website helpful, as they provide several resources
. If you are unfamiliar with tracheostomies, you might find their NTSP Manual 2013
(PDF file) helpful.