I'm not sure if this is the answer that you're looking for, since I'm not too knowledgeable about this... but often producers/composers have a terms and conditions about whether or not you can use their music and where it can be uploaded, etc. For example, some producers only allow covers of their songs to be uploaded on nico nico douga. This is often where you downloaded the off-vocal (i.e. piapro or their website) or in the zipped file. If there isn't anything like that it would be best to ask for permission via email or twitter, etc. If I remember correctly, if the karaoke/song is on Piapro, you're allowed to use it, but you need to leave a message on that song's page that you're using it and thank the original artist.
Also some songs (such as Aimai Elegy) aren't allowed to be covered unless you have direct permission from the artists because they're professional works.
So be careful! It's a good thing to leave credits, but it is often not enough.
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but often producers/composers have a terms and conditions about whether or not you can use their music and where it can be uploaded, etc. For example, some producers only allow covers of their songs to be uploaded on nico nico douga. This is often where you downloaded the off-vocal (i.e. piapro or their website) or in the zipped file. If there isn't anything like that it would be best to ask for permission via email or twitter, etc. If I remember correctly, if the karaoke/song is on Piapro, you're allowed to use it, but you need to leave a message on that song's page that you're using it and thank the original artist.
Also some songs (such as Aimai Elegy) aren't allowed to be covered unless you have direct permission from the artists because they're professional works.
So be careful! It's a good thing to leave credits, but it is often not enough.