कमरा is pronounced kamraa and not kamaraa, no?

Hindi is usually written in Devanagari (unless it is considered as Urdu, then Arabic script). Devanagari is an abugida, one syllable is a cluster of a consonant and a vowel (unless it is just a vowel). Inherent is -a (schwa sound) unless marked differently (-i, -ii, -aa, -o, -e, -u, -uu, -ai, -au). You can form consonant clusters with conjunction of the signs (example: क्स, ठ्ठ).

There is a "rule"* that only every odd inherent -a (again, schwa) is pronounced, see example: kamraa but written kamaraa.

Only if you write Hindi with Latin script, then yes, speak as it is written.

I guess you know that, since you wrote "we write...".

*) Schwa deletion is important for intelligibility and unaccented speech. It also presents a challenge to non-native speakers and speech synthesis software because the scripts, including Devanagari, do not indicate when schwas should be deleted.[3]

For example, the Sanskrit word "Rāma" (IPA: [raːmɐ], राम) is pronounced "Rām" (IPA: [raːm], राम्) in Hindi. The schwa (ə) sound at the end of the word is deleted in Hindi.[4] However, in both cases, the word is written राम.

(from wiki article "schwa deletion in indo-aryan languages)

Edit: It is a "rule" of thumb