How is "せんえん" pronounced?

[alo]

There’s a discussion here that might be helpful:

The Japanese N? ん

Another thing to point out with ん is that it isn’t always pronounced as an “N” sound. In words that have an ん followed by a ば or ぱ result in an M sound. For example, words like 先輩 (せんぱい） or 三倍 (さんばい）, the ん is actually pronounced as an M sound rather than an N sound. The basic idea is that whenever ん is followed by another syllable, you shape your mouth so as to say that next syllable and then make a nasal sound for the ん before pronouncing the next syllable. For vowels, this ends up transl…