helst on its own is the superlative of the word gärna: gärna, hellre, helst. You don't really have an exact counterpart in English – gärna is a lot like gladly or I'd love to and both hellre and helst translate into rather. Or preferably.
But the combination när som helst means 'at any time' or 'any time now'.
but why, "som" is pronounced as if s would be followed by a vowel like y,i,e, à or ò?
It's OK and it's been added now. We often use the present to mean future in Swedish. In the course, the idea is to accept it wherever it seems likely that the sentence could be about the future, which is certainly the case here. But we haven't always added all combinations, so just report them by using the 'my answer should be accepted' button.