There would be no ambiguity in your first example, “e le taks” cannot be but “et les taxes“, as “les” and “le” are pronounced differently (not to mention that in singular, that would be “la taxe“).

The second example transcript is “Maintenant, commencez !“.

There is no simple way to convert what your hear phonetically to words and sentences, although this is what computers are trying to do with varying results.

Only experience will give you the ability to train your ear to french phonems and then understand spoken French.

This would be the case with most and probably all languages anyway.

A few points:

• when you hear a language you know sense comes across before phonology. A common example that came from language learning is telling a 5 year-old “The boy who has a red sweater is going to school”, who repeated it as “the red boy is going to school” because he could internalize the phrase but no express the relative himself
• most languages are not pronounced as they are written, you seem to be pretty fluent in English, so you probably know it’s also a pretty bad offender in that respect, http://en.wikipedia.org/wiki/Ghoti being the canonical example, this poem is funnier http://www.spellingsociety.org/journals/j17/caos.php .
• liaisons and final consonants are inherited from 1500s French (http://en.wikipedia.org/wiki/Phonological_history_of_French) when all final vowels were still pronounced. When a French person writes ‘droit’, because of derivatives (droite, droiture, adroit, adroite) and liaisons, there’s no way the word isn’t written with a final “t”, so when you speak French it kind of makes sense.
• reading is a very good indicator of literacy, if you read a lot of French you will know that there is no ‘droi’ ‘droix’, ‘drois’, ‘droua’ or in your example ‘tax’,’taks’. When you look for a word from sound, only 2-3 written forms are in the language when many more could be possible with phonological rules.
• when someone speaks fast, some parts of the words are dropped. Which parts are important kept depends on the language and region, but Paris French does this a lot. “Maintenant” is pronounced “Maint-nan” instead of “Main-teu-nan”, the worst example I can think of is “c’est parce que” pronounced “sspasskeu” when speaking fast. You have to get used to it by hearing it, I don’t think there are any hard rules for it.

So not really a French specific answer, but the more you study, the more it comes together, with all the factors above.