Learn Portuguese (Portugal): A rádio anunciou que o capital desapareceu. - The radio station announced that the capital money disappeared.

This is a dual-gender noun where the meaning changes based on the article used. A rádio refers to the radio station or the broadcasting industry. If we used o rádio, it would mean the physical radio device itself. Since a station makes announcements, we use the feminine form here.

What would it mean if the sentence said a capital instead of o capital?

The word capital completely changes meaning depending on its gender. O capital (masculine) refers to money, financial funds, or assets. A capital (feminine) refers to a capital city, like Lisbon or London.

Are the feminine and masculine versions of these dual-gender nouns pronounced differently?

No, the pronunciation of the noun itself remains exactly the same! The only way you can tell them apart in spoken Portuguese is by listening for the articles (o versus a) or by checking any adjectives modifying them.

Both verbs (anunciou and desapareceu) are in the preterite (simple past). Why not use the imperfect (desaparecia) for the second verb?

We use the preterite desapareceu because the disappearance is viewed as a fully completed event that happened before the announcement. If we used the imperfect desaparecia, it would imply the money was actively in the process of disappearing right as the radio was announcing it.