Let’s start off with the three following simple examples:
Er betätigte zwei Tasten, sodass der Alarm ausgelöst wurde.
Er betätigte zwei Tasten so, dass der Alarm ausgelöste wurde.
Er betätigte zwei Tasten, so, dass der Alarm ausgelöste wurde.
In the first example, him pressing two keys causes the alarm; it does not matter which keys he pressed or how. In the second example, he needs to have pressed two particular keys or pressed them in a particular way for the alarm to go off. The third example is like the second one, but everything following the first comma is used parenthetically or after a break of thought – the first comma could as well have been a dash (Gedankenstrich). Note that those three cases are differentiated in spoken language by pauses at each comma.
Now, let’s look at your examples:
Sprechen Sie laut und deutlich, sodass jeder Sie versteht.
Sprechen Sie laut und deutlich so, dass jeder Sie versteht.
In the first case, speaking loud and clearly is supposed to cause everybody to understand you. In the second case, you are instructed to speak loud and clearly in such a way that everybody understands you. The example is however rather unidiomatic and I would rather say:
Sprechen Sie laut, deutlich und so, dass jeder Sie versteht.
Sprechen Sie laut und deutlich, und zwar so, dass jeder Sie versteht.
Your second example is actually correct the way you did it, as the following would imply that a given a can exist in many different ways and only in some of them, 2a would be x:
Es existiert ein a so, dass 2a =x.
While the above is all correct and rather simple, things are made complicate in two ways:
According to § 74 E1 of the spelling rules, the second comma in the third variant of the first example is optional, so you can also write:
Er betätigte zwei Tasten, so dass der Alarm ausgelöste wurde.
§ 39 E3 (2) of the spelling rules is explicitly allows writing so dass instead of sodass. I personally advise against this because it can be confused with the above and, going by stress and pauses, nobody speaks sodass as two words anymore – at least in my experience. When copy editing, I always change so dass to sodass. The Duden dictionary also lists sodass as preferable spelling. According to the old German spelling (prior to 1996/2006), only so dass was valid and some people may not have noticed that this has changed (or are stubborn) and thus state incorrectly that sodass is wrong.