What is the inlet/outlet speed of air going in/out of our mouth during inhalation/exhalation
Quick clarification firstly, the main change in thoracic volume that causes inspiration is not as a result of the alveoli expanding - they have no smooth muscle lining therefore are unable to spontaneously contract.  They do have some elastic fascia however their expansion is passive. The change is mainly as a result of the diaphragm contracting to become flatter, increasing the space that the lungs can occupy.  The external intercostal muscles also raise the ribs (in adults) which further allows for inspiration.  


Figure One: Inspiration and Expiration(1).  "Rib muscles" refers to the external intercostal muscles.

With that being said, the value I think you are looking for is peak expiratory flow.  Peak flow is a common medical measurement that is often taken in the management of patients with respiratory conditions, either as part of their management plan or acute assessment.  Peak flow is a measurement of the maximum speed that a person can exhale at.   

As the comments on your question suggest, this varies dramatically between person to person.  Ignoring any pathologies - such as asthma, chronic obstructive pulmonary diseases (COPD) or nerve damage to the diaphragm/intercostal muscles - peak expiratory flow varies primarily with age, gender and height.  These variations are summarised in charts of expected peak flow that are used, amongst other things, to grade the severity of an asthma attack (and consequently how aggressively to manage it).  Charts resemble the following:



Figure Two: Normal values for peak expiratory flow used in UK hospitals - The EU Scale (2)

So if you pick the height, gender and age that you feel is appropriate for your question, you can then read off peak flow in litres per minute.  Then you would need, I think, to work out the area of the aperture (in your case the mouth) to get a speed in metres per second?  




Medina B. Brenda's A&amp;P eportfolio: objective 48 &amp; 49: inhalation and exhalation [Online]. 2011 [accessed 2012 Dec 11]. Available from URL: http://blm1128.blogspot.co.uk/2011/04/objective-49-contrast-inspiration-and.html.
Häggström M. File: normal values for peak expiratory flow - EU scale.png [Online]. 2009 [accessed 2012 Dec 11]. Available from URL: http://en.wikipedia.org/wiki/File:Normal_values_for_peak_expiratory_flow_-_EU_scale.png.