I have a dental variety of /n/ in my own language. It is neither more or less like /l/ than any other coronal nasal. Of course sound changes like n > l or l > n are possible and known to have happened here and there.
Bengali is "close" to Santali in geographical terms, but not in terms of historical derivation. A look at the Bengali numeral system will tell you that it is of Indo-Aryan (and Indo-European) origin. The numerals for "nine" in all modern Indo-Aryan languages derive regularly from Proto-IA *nava (= Skt. nava) < PIE *newn (related to Latin novem, English nine, Welsh naw, Tocharian ñu, Albanian nëntë etc.), and the initial /n-/ is still there throughout IA, also in Bengali. Santali is a Munda language with a different set of numerals. The primary word for "nine" in Munda (and in Santali) is <are>. <lo> is a recent innovation in some Munda dialects. I would not exclude the possibility that it is a loan from Indo-Aryan with the initial denasalised. Some Munda languages have borrowed long series of IA numerals, e.g. Korwa has numerals of IA origin for the numbers 4-10 (includig <nau> '9'), and I'm fairly certain that Bihor <la:>, which is a member of a similar series, is indeed *nava in disguise. Experts in Munda etymology should be asked first. Maybe they know where <lo> comes from. Anyway, it is not an ancient pan-Indian numeral but a recent borrowing from Indo-Aryan into Santali at best. They are not _genetically_ related.
----- Original Message -----
From: kalyan97
To: phoNet@yahoogroups.com
Sent: Wednesday, February 27, 2002 3:54 AM
Subject: [phoNet] Re: Phonetic change lo -- no in some languages

--- In phoNet@y..., "Piotr Gasiorowski" <gpiotr@i...> wrote:
> Old Indo-Aryan (and common Iranian) nava and their Modern Indic and
Iranian reflexes (including Pashto n&h) are regular developments of
PIE *newn. (ending with a syllabic nasal, vocalised in Indo-Iranian).
Santali lo is simply unrelated.
But how is no~_ of Bengali explained? Is it a regular
development? Bengali is so close to Santali.

How about the l-n interchanges in some Indic languages?

Try saying the dental 'n' i.e. pronounced with the tongue against the
back of the upper front teeth (dental); it sounds like an 'l' when
the nose is not involved. Could this explain the nasals in Lahn.da
and Punjabi no_~ also meaning 'nine'. We don't know much about the
etymology of 'lo' in Santali to say if it is unrelated to IE; do we?