When a particle has the same vowel sound as a word right next to it, are you supposed to blend the vowel sounds together or say them each distinctly? I hear it both ways in the audio clips and I can’t figure out the rules. For example;

わたしたち　の　王  が　大すき　ですよ！ I hear her make a clear distinction between the o sound from the particle no and the o sound from king. But in this sentence;

火山　に　いきました。 I don’t find any separation between the ni and ikimashita. It sounds like “nikimashita” to me.

Is there a rule of thumb I can use for this? Thanks so much!