In Spanish, the “b” and the “v” are almost the same sound. They are both pronounced like an English “b”. So, for example, when you say the word for “wine”, “vino” you should really pronounce it “bino”. The English “v” sound does not exist in Spanish.
If a word begins with a “v” it is identical to the English “b” sound. However, if the “v” occurs in the middle of a word, or after an “m” or “n”, it is a softer sound, which we do not have in English. It is more of a combination sound between a “v” and a “b”, a softer “b”. It is like an English “v” but with the two lips touching instead of the lower lip and teeth.
Because the “b” and “v” sound the same in Spanish, this causes spelling problems for both Spanish people as well as people learning Spanish. Therefore you often see certain words spelt differently. For example, “cebiche” is sometimes spelt “ceviche”, (a shellfish dish).
So when spelling words out loud, to distinguish between the sounds you often hear the letter “b” “be alta” or “be larga” and the “v” as “be baja” or “be corta”.
I know from experience that not pronouncing a word in the correct way can lead to misunderstandings. When I first started living in Spain, I remember not pronouncing a word spelt with “v” as a “b” sound and I was therefore not understood. However, as soon as I pronounced it with a “b” sound, I was understood straight away.