"We are not wanting two books."
Translation:Chan eil sinn ag iarraidh dà leabhar.
The same reason why r in (Southern England) English car the r doesn’t make the Gaelic r sound. Or why k and gh in English knight don’t make the Gaelic c and gh sounds. ;-) Languages change and the orthography often stays behind.
As tj4232 wrote, it used to be pronounced as any other bh sound, but then some lenited sounds (mh, bh, gh, dh) in some environments tended to disappear between after a vowel, lengthening it or changing it into a diphthong. And what’s worse, it seems it happened a bit differently in different dialects. ;-)
See eg. Those pesky BH, DH, GH and MH section of The Unofficial Guide to Pronouncing Gaelic and if you want a more in-depth but IPA-heavy and harder to consume resource, the Akerbeltz’s printable guides to Gaelic spelling (there are links to pdfs in the wiki page).
For those reading from the app, the links are : https://cuhwc.org.uk/page/unofficial-guide-pronouncing-gaelic And: http://akerbeltz.org/index.php?title=A_printable_pronunciation_guide_to_Gaelic_spelling