Learn German: Nach dem Lied war der Jubel laut. - After the song, the cheering was loud.

Because the sentence starts with the prepositional phrase Nach dem Lied. In German main clauses, the conjugated verb must always sit in the second position (the V2 rule). Since the first position is taken by the prepositional phrase, the subject der Jubel must slide into the third position, immediately following the verb.

Why is it dem Lied instead of das Lied?

The preposition nach always triggers the dative case. Since das Lied is a neuter noun, the article changes to dem in the dative. This is a quick refresher that certain prepositions dictate the case of the nouns that follow them regardless of the verb in the sentence.

Is der Jubel an uncountable noun like "cheering" is in English?

Exactly. der Jubel is singular and uncountable. If you need a plural form to describe individual cheers or shouts, you would use a compound noun like die Jubelrufe.

How would I connect a clause to this sentence using nur dass to say "except that it was too short"?

You would add a comma followed by the subordinate clause, placing its conjugated verb at the very end: Nach dem Lied war der Jubel laut, nur dass er zu kurz war. Because nur dass is a subordinating conjunction, it sends the verb war to the end of its clause while restricting the positive statement made in the main sentence.