Maybe if you’re about to upload them on Soundcloud you’d better add a watermark to your tracks so to protect them from piracy.
Another option might be uploading them in “private mode” and send them the secret link.
Or you can use a service like WeTransfer. I would guess most libraries don’t even open a mail with attachments from a unknown mail adress, if they should get through the spam filter.
If there’s no obvious submission link,use their contact email and send them a short message (including a few credits,short bio) with a soundcloud link.
I had several positive replies doing this.
I wouldn’t like to send a downloadable link (WeTransfer etc) to someone that didn’t ask for it.
yeah i second Kubed’s approach. but actually if there is a phone number dont be afriad to pick up the phone and call them. you’d be surprised how many libraries are delighted to hear from new composers and i think they kinda appreciate the ambition that you were willing to call them.