In general, the magnetic field on your phone speaker is too weak to cause too many problems. So it shouldn't cause any damage to the magnetic strip on your credit card.
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There is no evidence to suggest that phones can deactivate hotel keys. Hotel keys typically use magnetic stripes or RFID technology to function, and neither of these technologies are affected by phones. Additionally, hotels would not use technology that is so easily disrupted by common electronic devices like phones.
Cards with scratches, exposure to debris, and other types of physical damage can be rendered unusable. Key cards may become inoperable when the magnetic stripe is demagnetized through exposure to magnets. Name tags, signs, and purse and wallet closures, and mobile phones are all potential sources of magnet exposure.
Experts say you also don't need to worry about turning your key card in at the end of your stay; the information expires so the hotels can reuse the cards.