For example :日本語を勉強します.
Do you pronounce ni hon go wo or ni hon go---
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“を” is pronounced in the same way as “お,” that is, without a consonant. Therefore, if it is preceded by a mora with vowel /o/, it sounds in the same way as chōonpu “ー.”
Some people pronounce “を” as /wo/, but this pronunciation is nonstandard.
「を」 is never used to lengthen a 「お」 sound, only 「う」 or 「ー」, and in certain specific situations 「お」.
Having said that, the "w" sound is normally elided so that it sounds similar to a long 「お」, but there is usually enough of a difference to make a distinction.
In other languages (I don't know the term for this) 語を can be transcribed as
There is a slight stop between both sound, almost not perceptible. In faster speech, if no cut is perceptible, there is definitely an accentuation difference. (or voice level)
To compare with other similar sounds:
The other answers seem to have come controversy, so I'm going to propose a different approach, which is that of a non-native learner of the language. Native speakers will disagree about the pauses and intonations, just like native English speakers disagree over things like "tomato".
The point to know, in reference to your question, is that there is no convention or rule that makes
を combine with a word before it.
However, when listening to native speakers talk, as a non-native, you will very likely not hear any distinction, especially if they are speaking quickly. And, of course, any kind of distinction you might discern will vary depending on who you listen to, so you can't expect it every time.
My recommendation to you as a learner of the language is that when you speak, consciously keep
を separate from the word before. Pause if you want, don't if you don't want. Also, whether or not your separation of
を from the word before it is distinct or not will not matter too much to a listener, because context makes it understood that it was there.
The important thing is that you know it's there. Because good grammar is not just a good idea, it's the law.