"The security guards began to eat sandwiches."
Translation:Ochroniarze zaczęli jeść kanapki.
I think that "Strażnik" can be any type of guard, including a medieval guard, or a guard in a prison. "Ochroniarz" is either an employee of a security company, or a hired bodyguard.
The text says 'zaczęli', but the audio says 'zaczeli' (without the ‘en’ sound typically made by the ‘ę’ character).
Why is this?
You actually wouldn't pronounce the ę in this example that clearly, it would sound really strange.
ęl and ęł sound just like el and eł.
Same for ął which is rather like oł (zaczął -> 'zaczoł'). I don't know if ąl exists in any word.
Is there any way to recognise these Polish words that are pronounced differently to how they are written?
Or are they just rare exceptions like the English words ‘knife’ and ‘chemist’?
What Jellei wrote, is a strict rule and not an execption. Words that do not follow any rule are indeed extremely rare and there is no way to recognize them.
But what is the rule?
For example, if ‘ęl’ and ‘ęł’ are always pronounced ‘el’ and ‘eł’, then why are they not written that way?
The nasal sound ę is part of the past-stem of zacząć. It is featured in all forms that contain that stem, for example in zaczęty (passive adjectival participle), zaczęto (impersonal past), zaczęcie (verbal noun). It's just that ł and l have the property of denasalizing all nasal vowels.
With the rule that you propose you would have to watch out for ł/l so you that don't write an ę before them. With the rule in place, you'll have to watch out not to pronounce an ę before ł/l.
So both rules would be logical, with the second (real) one being preferable, since vowel alternations (especially denasalization) feels more intuitive in speech than in writing.